chacha: Use Overlapping in the implementation of the fallback impl.

[briansmith]

Eliminate all of the unsafe in the fallback implementation.

[codecov]

Codecov Report

Attention: Patch coverage is 79.79798% with 20 lines in your changes missing coverage. Please review.

Project coverage is 97.02%. Comparing base (fa0c22b) to head (c956545).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/aead/overlapping/base.rs	73.17%	11 Missing ⚠️
src/aead/overlapping/array.rs	77.41%	6 Missing and 1 partial ⚠️
src/aead/chacha/fallback.rs	85.71%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2211      +/-   ##
==========================================
- Coverage   97.10%   97.02%   -0.08%     
==========================================
  Files         159      160       +1     
  Lines       20326    20391      +65     
  Branches      457      458       +1     
==========================================
+ Hits        19738    19785      +47     
- Misses        482      498      +16     
- Partials      106      108       +2     

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