Fixed build issue when compiling for 32-bit ARM platforms under Visual Studio.

Fixed "permissive" issue under Visual Studio whereas some users would get link errors with some flags due to static constexpr not being considered inline by default under Visual Studio.

In this version, as in the C++17 standard, we forbid leading spaces as well as the leading '+' character.

NaN/infinity parsing has been revised. In particular, parse "nan(n-char-seq-opt)" as required by C++17 20.19.3.7, credit @eugenegff.

Update to CMake to allow package installation @georg-emg

Fixes a potential issue raised by Ivan Smirnov and improved minutely code clarity.

Minor fix in the fall back code (issue 47).

Better support for clang under msys2.

• Better support for 32-bit systems (@biojppm)
• Removed iostream dependency (@biojppm)
• The algorithmic fast paths are more common
• Adopted slightly faster SWAR code (@aqrit)