Switch to a recursive descent parser

[kodek16]

Some of the other issues would be very tricky to handle with the currently used parser generator (LALRPOP). The LR grammar is already quite inflated because of technical reasons (dangling else prevention), and adding some features like f-strings (#14) or optional semicolons (#15) would be either very difficult or downright impossible to fit into a pure LR grammar.

A long-term solution would be to move to a custom recursive descent parser that handles syntactic ambiguities manually one-by-one. A parser combinator might be useful to that end.