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>