Improve std adapters, fix problems triggered in amalgamate.py

[biojppm]

• Add traits class c4::is_writeable_string<>
• c4/std/*.hpp adapters: add missing forward declarations of c4::is_string<>
• Add default-less csubstr::copy_from() to simplify implementations of to_chars()/from_chars()
• Minor internal changes in c4::substr
• Add C++20 std::span<char> and std::span<const char> adapters in new header c4/std/span.hpp

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.91%. Comparing base (68de1b6) to head (5664ef3).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #156      +/-   ##
==========================================
- Coverage   96.91%   96.91%   -0.01%     
==========================================
  Files          28       28              
  Lines        3149     3141       -8     
==========================================
- Hits         3052     3044       -8     
  Misses         97       97              

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[coveralls]

Pull Request Test Coverage Report for Build 23031680485

Details

• 23 of 23 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.005%) to 96.857%

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Totals
Change from base Build 22970124683:	-0.005%
Covered Lines:	3113
Relevant Lines:	3214

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💛 - Coveralls