Rewrite of Midi.c in Python (issue 7016046)

[pkx166h]

Reviewers: benrg,

Message:
Actually, I didn't rewrite it - as the comment above suggests - this was
from Ben Rudiak-Gould.

I am helping him at least get some discussion review of his code
submission.

This patch has not been patchy-tested and I have just included
everything (i.e. also the .mid files) that were provided by Ben. I have
put them in the place that I thought might be appropriate but I don't
really know if this is right or not.

Description:
Rewrite of Midi.c in Python

I rewrote midi.c in Python and tested it on the largest midi file in
IMSLP [1]. On my laptop it takes about 200 ms instead of 50 ms to load
the file, but that's a small fraction of midi2ly's four-second
runtime, and there are a lot of optimizations that can be done in
midi2ly itself. (For example, memoizing the creation of Duration
objects saves about 500 ms.) I think getting rid of the sole occupant
of usr/lib/lilypond/current/python is worth it.

Please review this at https://codereview.appspot.com/7016046/

Affected files:
  A scripts/midi.py
  A scripts/test-all-valid-sequences.mid
  A scripts/test-error-empty.mid
  A scripts/test-error-initial-repeat.mid
  A scripts/test-error-mthd-bad-magic.mid
  A scripts/test-error-mthd-length-5.mid
  A scripts/test-error-mthd-truncated-1.mid
  A scripts/test-error-mthd-truncated-2.mid
  A scripts/test-error-mthd-truncated-3.mid
  A scripts/test-error-mtrk-bad-magic.mid
  A scripts/test-error-mtrk-truncated-1.mid
  A scripts/test-error-mtrk-truncated-2.mid
  A scripts/test-error-mtrk-truncated-3.mid