The methods for fallible slice allocation in a given allocator were missing, which was an oversight according to rust-lang/wg-allocators#130

This PR adds them as `try_new_uninit_slice_in` and `try_new_zeroed_slice_in`.

Also adds missing punctuation to the doc comments of ` try_new_uninit_slice` and `try_new_zeroed_slice`

Apologies for the many attempts, my dev loop for this consists of editing on github, committing, and then waiting for the CI failure log to yell at me.