Install MSVC toolchains

[uranusjr]

More inclined to make this an optional feature because even downloading the minimum build tools is pretty large, like, in GB territory.

Probably something like

snafu install 3.6 --with-toolchain

And a snafu component command to install arbitrary things:

snafu component install 3.6-toolchain

(Component and flag names are subject to change.)

[uranusjr]

https://wiki.python.org/moin/WindowsCompilers https://en.wikipedia.org/wiki/Microsoft_Visual_C%2B%2B#Internal_version_numbering

So

Python	MSVC	Visual Studio
2.7	9.0	2008
3.4	10.0	2010
3.5 3.6	14.0	2015

According to Steve Dower MSVC 14.1 (VS2017) is “theoretically binary compatible” with 14.0 (VS2015), so let’s use that for 3.5 and 3.6 as well. I’m guessing 3.7 will be compiled against 14.1.

Random notes:

• The installer of Build Tools for VS2017 requires .NET Framework 4.6. KB3151800 (4.6.2 offline installer) should cover this.
• Not sure about requirements of VS2010 and 2015. Will need to check.
• I think we don’t need to mess with environs and vcvarsall.bat now, since Pip and Setuptools do all the toolchain discovery now, right? Right? Please? (And #3 to make sure the user gets it if they do.)