Add `trim_prefix` and `trim_suffix` methods for both `slice` and `str` types.

Implements `trim_prefix` and `trim_suffix` methods for both `slice` and `str` types, which remove at most one occurrence of a prefix/suffix while always returning a string/slice (rather than Option), enabling easy method chaining.

## Tracking issue
rust-lang#142312

## API
```rust
impl str {
    pub fn trim_prefix<P: Pattern>(&self, prefix: P) -> &str;
    pub fn trim_suffix<P: Pattern>(&self, suffix: P) -> &str
    where
        for<'a> P::Searcher<'a>: ReverseSearcher<'a>;
}

impl<T> [T] {
    pub fn trim_prefix<P: SlicePattern<Item = T> + ?Sized>(&self, prefix: &P) -> &[T]
    where
        T: PartialEq;
    pub fn trim_suffix<P: SlicePattern<Item = T> + ?Sized>(&self, suffix: &P) -> &[T]
    where
        T: PartialEq;
}
```

## Examples
```rust
// Method chaining
assert_eq!(" <https://example.com/> ".trim().trim_prefix('<').trim_suffix('>').trim(), "https://example.com/");

// Slices
let v = &[10, 40, 30];
assert_eq!(v.trim_prefix(&[10]), &[40, 30][..]);
```

## ACP
Originally proposed in rust-lang/libs-team#597

Rollup of 6 pull requests

Successful merges:

 - #141990 (Implement send_signal for unix child processes)
 - #142331 (Add `trim_prefix` and `trim_suffix` methods for both `slice` and `str` types.)
 - #142502 (rustdoc_json: improve handling of generic args)
 - #142629 (Add config builder for bootstrap tests)
 - #142687 (Reduce uses of `hir_crate`.)
 - #142714 (add comment to `src/bootstrap/build.rs`)

r? `@ghost`
`@rustbot` modify labels: rollup

Add `trim_prefix` and `trim_suffix` methods for both `slice` and `str` types.

Implements `trim_prefix` and `trim_suffix` methods for both `slice` and `str` types, which remove at most one occurrence of a prefix/suffix while always returning a string/slice (rather than Option), enabling easy method chaining.

## Tracking issue
rust-lang#142312

## API
```rust
impl str {
    pub fn trim_prefix<P: Pattern>(&self, prefix: P) -> &str;
    pub fn trim_suffix<P: Pattern>(&self, suffix: P) -> &str
    where
        for<'a> P::Searcher<'a>: ReverseSearcher<'a>;
}

impl<T> [T] {
    pub fn trim_prefix<P: SlicePattern<Item = T> + ?Sized>(&self, prefix: &P) -> &[T]
    where
        T: PartialEq;
    pub fn trim_suffix<P: SlicePattern<Item = T> + ?Sized>(&self, suffix: &P) -> &[T]
    where
        T: PartialEq;
}
```

## Examples
```rust
// Method chaining
assert_eq!(" <https://example.com/> ".trim().trim_prefix('<').trim_suffix('>').trim(), "https://example.com/");

// Slices
let v = &[10, 40, 30];
assert_eq!(v.trim_prefix(&[10]), &[40, 30][..]);
```

## ACP
Originally proposed in rust-lang/libs-team#597

Rollup of 10 pull requests

Successful merges:

 - #138291 (rewrite `optimize` attribute to use new attribute parsing infrastructure)
 - #141990 (Implement send_signal for unix child processes)
 - #142331 (Add `trim_prefix` and `trim_suffix` methods for both `slice` and `str` types.)
 - #142478 (install docs for each target in different directory)
 - #142571 (Reason about borrowed classes in CopyProp.)
 - #142668 (vec_deque/fmt/vec tests: remove static mut)
 - #142687 (Reduce uses of `hir_crate`.)
 - #142690 (expand: Remove some unnecessary generic parameters)
 - #142699 (Update books)
 - #142714 (add comment to `src/bootstrap/build.rs`)

Failed merges:

 - #142502 (rustdoc_json: improve handling of generic args)

r? `@ghost`
`@rustbot` modify labels: rollup

Rollup of 9 pull requests

Successful merges:

 - #142331 (Add `trim_prefix` and `trim_suffix` methods for both `slice` and `str` types.)
 - #142491 (Rework #[cold] attribute parser)
 - #142494 (Fix missing docs in `rustc_attr_parsing`)
 - #142495 (Better template for `#[repr]` attributes)
 - #142497 (Fix random failure when JS code is executed when the whole file was not read yet)
 - #142575 (Ensure copy* intrinsics also perform the static self-init checks)
 - #142650 (Refactor Translator)
 - #142713 (mbe: Refactor transcription)
 - #142755 (rustdoc: Remove `FormatRenderer::cache`)

r? `@ghost`
`@rustbot` modify labels: rollup

Rollup of 9 pull requests

Successful merges:

 - #142331 (Add `trim_prefix` and `trim_suffix` methods for both `slice` and `str` types.)
 - #142491 (Rework #[cold] attribute parser)
 - #142494 (Fix missing docs in `rustc_attr_parsing`)
 - #142495 (Better template for `#[repr]` attributes)
 - #142497 (Fix random failure when JS code is executed when the whole file was not read yet)
 - #142575 (Ensure copy* intrinsics also perform the static self-init checks)
 - #142650 (Refactor Translator)
 - #142713 (mbe: Refactor transcription)
 - #142755 (rustdoc: Remove `FormatRenderer::cache`)

r? `@ghost`
`@rustbot` modify labels: rollup

Rollup merge of #142331 - deven:trim_prefix_suffix, r=Amanieu

Add `trim_prefix` and `trim_suffix` methods for both `slice` and `str` types.

Implements `trim_prefix` and `trim_suffix` methods for both `slice` and `str` types, which remove at most one occurrence of a prefix/suffix while always returning a string/slice (rather than Option), enabling easy method chaining.

## Tracking issue
#142312

## API
```rust
impl str {
    pub fn trim_prefix<P: Pattern>(&self, prefix: P) -> &str;
    pub fn trim_suffix<P: Pattern>(&self, suffix: P) -> &str
    where
        for<'a> P::Searcher<'a>: ReverseSearcher<'a>;
}

impl<T> [T] {
    pub fn trim_prefix<P: SlicePattern<Item = T> + ?Sized>(&self, prefix: &P) -> &[T]
    where
        T: PartialEq;
    pub fn trim_suffix<P: SlicePattern<Item = T> + ?Sized>(&self, suffix: &P) -> &[T]
    where
        T: PartialEq;
}
```

## Examples
```rust
// Method chaining
assert_eq!(" <https://example.com/> ".trim().trim_prefix('<').trim_suffix('>').trim(), "https://example.com/");

// Slices
let v = &[10, 40, 30];
assert_eq!(v.trim_prefix(&[10]), &[40, 30][..]);
```

## ACP
Originally proposed in rust-lang/libs-team#597

Add `trim_prefix` and `trim_suffix` methods for both `slice` and `str` types.

Implements `trim_prefix` and `trim_suffix` methods for both `slice` and `str` types, which remove at most one occurrence of a prefix/suffix while always returning a string/slice (rather than Option), enabling easy method chaining.

## Tracking issue
rust-lang#142312

## API
```rust
impl str {
    pub fn trim_prefix<P: Pattern>(&self, prefix: P) -> &str;
    pub fn trim_suffix<P: Pattern>(&self, suffix: P) -> &str
    where
        for<'a> P::Searcher<'a>: ReverseSearcher<'a>;
}

impl<T> [T] {
    pub fn trim_prefix<P: SlicePattern<Item = T> + ?Sized>(&self, prefix: &P) -> &[T]
    where
        T: PartialEq;
    pub fn trim_suffix<P: SlicePattern<Item = T> + ?Sized>(&self, suffix: &P) -> &[T]
    where
        T: PartialEq;
}
```

## Examples
```rust
// Method chaining
assert_eq!(" <https://example.com/> ".trim().trim_prefix('<').trim_suffix('>').trim(), "https://example.com/");

// Slices
let v = &[10, 40, 30];
assert_eq!(v.trim_prefix(&[10]), &[40, 30][..]);
```

## ACP
Originally proposed in rust-lang/libs-team#597