Make the output of otlLib.buildLigatureSubst() deterministic.
Before this change, otlLib ligatures were emitted in a non-deterministic
sorting order (depending on what hash value Python would give
to a string tuple).
Contains the sketch and start of a stateless GSUB/GPOS/GDEF builder.
Will be used by mtiLib, feaLib, and the future voltLib. Can also
be used by clients to build these tables with much less pain.
