Add c4::is_string<T>

[biojppm]

... and other minor changes:

• Add c4::is_string<T> trait in c4/substr.hpp, and appropriate definitions for std::string, std::string_view and std::vector<char>. Default c4::dump_directly<T> to c4::is_string<T>.
• Minor cleanup to c4::srcloc in c4/error.hpp
• Minor implementation changes in c4::mem_repeat()
• Suppress false-positive null-dereference warnings for GCC >6
• Update fast_float to 8.2.4

[codecov]

Codecov Report

❌ Patch coverage is 96.42857% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.91%. Comparing base (828c552) to head (a1b3a05).
⚠️ Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
src/c4/language.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #155   +/-   ##
=======================================
  Coverage   96.91%   96.91%           
=======================================
  Files          28       28           
  Lines        3144     3149    +5     
=======================================
+ Hits         3047     3052    +5     
  Misses         97       97           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[coveralls]

Pull Request Test Coverage Report for Build 22965107324

Details

• 27 of 28 (96.43%) changed or added relevant lines in 9 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 96.862%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/c4/language.cpp	0	1	0.0%

Totals
Change from base Build 21361247058:	0.005%
Covered Lines:	3118
Relevant Lines:	3219

---

💛 - Coveralls