Adding core closure operations for FSTs

[LiamSchilling]

Features:

• FST trimming, inversion, concatenation, Kleene closure, union, composition, determinization, onwardization, and minimization
• Additional variations of rewrite and transition functions
• FSTs may have descriptive names, FST operations combine them appropriately
• Explicit marking of invariants and postconditions

In progress:

• Tests of client-facing features