Remove high-level `HasField::project_inner`, and rename
`HasField::project_raw` to `HasField::project`.

The new plan is for `pointer::cast::Project` and `HasField` to only have
`project` methods which project from `PtrInner<Src>` to `*mut Dst`, and
for callers who want `PtrInner -> PtrInner` or `Ptr -> Ptr` to use
inherent methods on `PtrInner` and `Ptr`. Compared to a default-impl'd
method on `HasField`, this reduces indirection and reduces pressure on
the optimizer.

Reverts #2884, #2885

gherrit-pr-id: G481927cb6e9ca76c1531c05465f608e1b2607a62