These items allow to make inherent impls for `*const str` and `*mut str`.

This patch adds the following methods to `*const str` and `*mut str`:
- `len`
- `as_ptr` (`as_mut_ptr`)
- `get_unchecked` (`get_unchecked_mut`)

Similar methods have already existed for raw slices.

This patch adds the following methods to `NonNull<str>`:
- `len`
- `as_non_null_ptr`
- `as_mut_ptr`
- `get_unchecked_mut`

Similar methods have already existed for raw slices, raw strings and nonnull
raw slices.

This commit changes documentation of the following methods as proposed
by the PR review
- `<*const [T]>::len`
- `<*mut [T]>::len`
- `<*const str>::len`
- `<*mut str>::len`
- `<*const str>::get_unchecked`
- `<*mut str>::get_unchecked_mut`
- `NonNull::<str>::get_unchecked_mut`