Use distribution of mpy-cross as default for %mpy-cross command

[andrewleech]

So I've gone and made an auto-compiler for mpy-cross:
https://gitlab.com/alelec/mpy_cross

It builds copies of mpy-cross for osx, linux (32 and 64 bit) and windows (32 and 64 bit).
Every micropython tag and master once a week.

Tag releases are bundled into python wheels and released on pypi, ready for use here :-)
https://pypi.python.org/pypi/mpy-cross

This PR brings in mpy_cross as a dependency and then uses the bundled binary by default.
This can still be overridden with the original set-exe if needed.

Also now supports '*' in filename to run on multiple files

[goatchurchprime]

How did you solve this problem? micropython/micropython#3410

[andrewleech]

Ah, well it's not exactly solved... however my builder has released copies for all tagged builds of micropython.

As such, if you're using an old version of micropython you could install a matching version of mpy-cross.

Similarly if there's a new syntax change in master, I've got regular builds from master available on the gitlab page, so you could an update.

I don't have any method in place to detect when an incorrect version is used however with the current versions it should tell us itself? micropython/micropython#3357 (comment)