fix str mutating through a ptr derived from &self
Found by Miri: In `get_unchecked_mut` (also used by the checked variants internally) uses `str::as_ptr` to create a mutable reference, but `as_ptr` takes `&self`. This means the mutable references we return here got created from a shared reference, which violates the shared-references-are-read-only discipline!
For this by using a newly introduced `as_mut_ptr` instead.
ecb6503169