The functions are under feature gate `str_from_raw_parts` and are similar to
`slice_from_raw_parts`, `slice_from_raw_parts_mut`.

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>`:
- `str_from_raw_parts`
- `len`
- `as_non_null_ptr`
- `as_mut_ptr`
- `get_unchecked_mut`

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