WIP: Ply Parser

Replace the SPARK parser with one using PLY

TODO (in decreasing priority):
 * Fix Sqrt formatting (`Sqrt[x]` presently formatted as `x^(1 / 2)`).
 * Fix misc. pattern parsing issues.
 * Figure out what's going wrong when loading `Import` definitions. 
 * Map `DifferentialD` token to `dx` during `TeXForm`.   (`additional_entities` ???)
 * Add some more parsing benchmarks (and write up comparison with SPARK parser).