Make this more generally useful and in line with what the mingw
and unix ports do: 16bit dig size to work on 32bit ports, a
self-contained qstrdefs.preprocessed.h because makemanifest.py
uses that, and a dev variant which effectively puts this to use:
previously the uasyncio module wasn't frozen but instead tests
ran by importing it from the extmod/ directory.
This is in line with the change made for other ports in d53c3b6a: since
the default output directory already includes the variant name in it
there's no need to add it to the executable as well.
The xmlns attribute is required for older msbuild version (e.g. for
VS2015). Add it where needed, and reorder the attributes so all
files look the same.
