Rewrite the HASH DRBG base class for C++20

[mborland]

Closes: #226

[codecov]

Codecov Report

Attention: Patch coverage is 99.27273% with 2 lines in your changes missing coverage. Please review.

Project coverage is 99.93%. Comparing base (6cafb99) to head (728e70e).
Report is 26 commits behind head on develop.

Files with missing lines	Patch %	Lines
include/boost/crypt2/drbg/detail/hash_drbg.hpp	98.98%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #234      +/-   ##
===========================================
- Coverage   100.00%   99.93%   -0.07%     
===========================================
  Files           78       81       +3     
  Lines         2948     3278     +330     
===========================================
+ Hits          2948     3276     +328     
- Misses           0        2       +2     

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