Add grammar/parser implementation

[adamchristiansen]

The lexer (#17) was designed to operate with lalrpop. Use lalrpop to implement the parser.

A hand-rolled parser will be implemented based on the AST proposed in #43 and the new lexer interface proposed in #44. This will likely be a recursive descent parser (possibly extended with a Pratt parser for operators).

Since the compiler will be bootstrapped (#47), we will use lalrpop.