Add configuration which otherwise has to be set via the UI so the file is
more self-contained, and remove configuration which is not needed because
it's the same as the default. The major change here is that for a while
now Appveyor has been using Visual Studio 2015 by default while we still
want to support 2013.
Add the project file to the mpy-cross directory, which is also where the
executable ends up, and change the Appveyor settings to build mpy-cross
with both msvc and mingw-w64 and verify this all works by running tests
with --via-mpy.
Build and test 32bit and 64bit versions of the windows port using gcc
from mingw-w64. Note a bunch of tests which rely on floating point
math/printing have been disabled for now since they fail.
This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.
