polyfill: Rewrite as_chunks/as_chunks_mut/flatten/flatten_mut.

[briansmith]

Define new types AsChunks<T,N> & AsChunksMut<T,N> that act like &[[T; N]] and &mut [[T; N]], respectively. Although there is a ton of boilerplate to do this, the advantage is we eliminate the unsafe in the implementations of as_chunks and as_chunks_mut, respectively.

It turns out we only have flatten/flatten_mut to undo the effects of as_chunks/as_chunks_mut. The new polyfills have a very natural implementation of flattening: just return the inner slice. Thus, we eliminate the prior use of unsafe in flattening easily.

core renamed flatten and flatten_mut to as_flattened and as_flattened_mut, respectively. Use the new naming.

Move as_chunks and related machinery into its own file, and do the same for as_chunks_mut. Then we can more easily compare the two implementations for (in)consistency.

diff src/polyfill/slice/as_chunks.rs \
     src/polyfill/slice/as_chunks_mut.rs

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.76%. Comparing base (0db51a8) to head (ae90b0a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2287      +/-   ##
==========================================
+ Coverage   96.72%   96.76%   +0.03%     
==========================================
  Files         171      173       +2     
  Lines       20843    20872      +29     
  Branches      489      487       -2     
==========================================
+ Hits        20161    20196      +35     
+ Misses        577      575       -2     
+ Partials      105      101       -4     

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