Previously, `is_bit_valid` had the signature:

```rust
fn is_bit_valid<A>(c: Ptr<'_, Self, (A, Unaligned, Initialized)>) -> bool
```

In this commit, we remove the `A` aliasing parameter and wrap `Self` in
`ReadOnly`, yielding:

```rust
fn is_bit_valid(c: Ptr<'_, ReadOnly<Self>, (Shared, Unaligned, Initialized)>) -> bool
```

This ensures that `is_bit_valid`'s argument is always uconditionally
`Immutable` regardless of whether `Self: Immutable`. This solves a
number of open problems:
- Ensures that `is_bit_valid` can never mutate its referent (#1831),
  which is important for custom validators (#1330)
- Makes it so that custom validators can be written without needing to
  be generic over aliasing, which in turn means we can support custom
  validators without exposing much of our `Ptr` machinery in our public
  API
- Allows us to support `#[derive(TryFromBytes)]` on unions without
  requiring `Self: Immutable` (#1832)
- Permits `T -> U` fallible transmutation in more cases (see #2336 for
  more details)

Makes progress on #2336

gherrit-pr-id: G7691845b6b02e9f3d9578435d732bacfa6ca674f