Implement parsing library

[joshcbrown]

We're using parser-combinator-y methods already to allow parsing across module boundaries. It would be nice to consolidate this with a combinator library that can keep track of global source position and abstract away the nitty-gritty details of current methods.

My main consideration here is that I want the library to be opt-in, such that if it feels too heavyweight you can just code a parser directly. A corollary of this is that migrating existing logic should be really easy (though some changes will definitely be necessary).

So far, I've laid the foundation and implemented a POC parser for SExp. The logic is pleasingly simple compared to the current parser.

The main things to do from here are:

• Improve error messages from parser. Annotate source in error a la Rust, Elm etc.
• Settle on new parsing signature, make method to lift existing code into new signature
• Solidify Parser module boundaries with interface file

Happy for any input on this direction