Minor MSVC fixes

[jlblancoc]

• -march=native is not for MSVC
• Nicer grouping of targets in IDE "folders"

[chrisbeall]

Oops! But we don't want Windows users to miss out on the huge speedup completely, so if there's no direct equivalent flag, then we should try to enable AVX, FMA, etc. directly? Something like this maybe?

CHECK_CXX_COMPILER_FLAG("/arch:AVX" COMPILER_OPT_ARCH_AVX_SUPPORTED)
if(COMPILER_OPT_ARCH_AVX_SUPPORTED)
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /arch:AVX")
endif()

[dellaert]

I will merge, I propose we leave the windows improvements for another PR.