Implement query parser

[martinm-nm]

Implement a query parser that uses the existing grammar and builds an AST. Work items:

• Redesign the Query class to be a monadic API instead - this will solve the problem of storing which of rollup and fill was intended to come first in the query (which we don't have a good way of capturing with the current API). After spiking this, I decided that instead of making a big change like that let's just make rollup and fill items in a list that can be provided in any order. It's so much less disruptive.
• Extend the AST to to cover not only queries but also query expressions (functions and formulas).
• [ ] Write a matcher for AST objects that's needed for testing.
• Write a parser with tests that parses query strings and returns ASTs. Use the matcher to test the parser.