Migrate from 'parsimonious' to 'grako'

[crantila]

We're currently using the parsimonious library for parsing search queries. While that seemed like a good idea, Grako is used more widely, maintained more actively, and probably faster. Plus the grammar is more flexible, and now that I'm using Grako in another project, I've seen that it's a better solution, and I don't need to spend Cantus time learning it!