For some gcc version (Ubuntu 5.5.0-12ubuntu1~16.04) the existing code
crashes when the minimum value of int64_t is outputted.

Resurrect the code from before 546e2cbf (🚨 fixed some warnings,
2019-03-13) but delegate the sign removal so that the compilers don't
complain about taking the negative value of an unsigned value.

In addition we also rewrite the expression so that we first increment
and then negate.

The definition of remove_sign(number_unsigned_t) is never called as
unsigned values are never negative.

Add gcc 9 and compile with experimental C++20 support

Use GNUInstallDirs instead of hard-coded path.

Package Manager: MSYS2 (pacman)

Add a package installation instructions for MSYS2 software distro.

Fix json.hpp compilation issue with other typedefs with same name (Issue #1642)