Implement quantization

[daveyarwood]

In MML, you can "quantize" notes, which sets the actual length of the note regardless of the placement. This is effectively just setting the amount of space between notes. MML's formula is a little obtuse IMHO. I think a more intuitive way would be to take a value 0-100 and use that as the percentage of the note that is heard, i.e. 100% is slurred notes (no space between notes), 0% is a rest for the duration of the note. 90% is probably a good default.

TODO: come up with a syntax* and add it to the grammar, write a test alda score that uses quantization.

*Should probably just let it be an attribute called "quant," for use during attribute changes. This seems clearer than copying MML's q<number> syntax.