The methods for fallible slice allocation in a given allocator were missing, which was an oversight according to https://github.com/rust-lang/wg-allocators/issues/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`