Generate rhythm

Add composition/rhythm package. The process for generating rhythm is:

1. Create a rhythm tree through recursive subdivision (see Node.SubdivideRecursive).
2. Traverse the tree, using a function to limit depth (see Generator.Make).

Move notation/rat to common/rat.
Move performance/function to common/function.

Score-to-MIDI

Basic MIDI functionality:

• write score to SMF using gomidi
• play score on default output using rtmidi