Lambda Calculus Macro `λ!` in Rust (github.com)

Next step, implement a `normalize` function for an `Expression`. Then syntactic sugar for all the primitive types.

A few critical TODOs, but neat so far :)

FWIW, I've moved this macro into https://github.com/nixpulvis/lalrpop-lambda, and it's got the start of a normalize function.