Merge branch 'develop' into json-view/02b-float-parser

Conflicts:
- number_parse.hpp: kept this branch's float parser, which replaces the
  Eisel-Lemire code that develop's side changed (#5750 made its digit
  counter unsigned; this parser has no such counter, and it compiles
  cleanly with GCC's -Wstrict-overflow=5).
- number_handling.md, template_parameters.md: kept this branch's
  description of the conversion and added develop's "Before version
  3.13.0" sentence.

Ran make amalgamate.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-locale-cpp.cpp

@@ -423,6 +423,7 @@ struct LocaleSwitchingStreambuf final : std::streambuf
     std::string locale_after_first_write;
     bool switched = false;
 
+  protected:
     std::streamsize xsputn(const char* s, std::streamsize n) override
     {
         if (!switched)