added cmake install targets

[georg-emg]

This patch adds install targets to the CMakeLists.txt, that will install the headers, plus a cmake module. This allows users to include fast_float in their cmake projects using the following:

find_package(FastFloat REQUIRED)
	
target_link_libraries(MyTarget PRIVATE FastFloat::fast_float)

I created this patch as a preparation for submitting fast_float as a port to Microsoft's vcpkg package manager.

[lemire]

Happy to merge this. I will bump the version number.