std: Align `raw` modules with unsafe conventions

This commit is an implementation of [RFC 240][rfc] when applied to the standard
library. It primarily deprecates the entirety of `string::raw`, `vec::raw`,
`slice::raw`, and `str::raw` in favor of associated functions, methods, and
other free functions. The detailed renaming is:

* slice::raw::buf_as_slice => slice::with_raw_buf
* slice::raw::mut_buf_as_slice => slice::with_raw_mut_buf
* slice::shift_ptr => deprecated with no replacement
* slice::pop_ptr => deprecated with no replacement
* str::raw::from_utf8 => str::from_utf8_unchecked
* str::raw::c_str_to_static_slice => str::from_c_str
* str::raw::slice_bytes => deprecated for slice_unchecked (slight semantic diff)
* str::raw::slice_unchecked => str.slice_unchecked
* string::raw::from_parts => String::from_raw_parts
* string::raw::from_buf_len => String::from_raw_buf_len
* string::raw::from_buf => String::from_raw_buf
* string::raw::from_utf8 => String::from_utf8_unchecked
* vec::raw::from_buf => Vec::from_raw_buf

All previous functions exist in their `#[deprecated]` form, and the deprecation
messages indicate how to migrate to the newer variants.

[rfc]: https://github.com/rust-lang/rfcs/blob/master/text/0240-unsafe-api-location.md
[breaking-change]

Closes #17863
I would rather not get too too long with the function name, but conventions-wise it may belong as that yeah. Note that the body of this function is quite small and lends itself to composition pretty well, so we may be able to remove it entirely if we don't like it.