This breaks out the pattern matching/unification algorithm from cl-interpret/interpret.rs to cl-interpret/pattern.rs
TODO: pattern destructuring in const, static :^)
- Everything is different now
- Builtins are now built on top of Rust functions, so they can be recursive!
- TODO: allow macro-defined builtins to call each other?
- The builtins! macro is a lot nicer to work with
- No redundant return value
- Maps the result over Into::into, allowing for type inference!
- Uses explicit pattern syntax instead of weird binding, where possible
- Does not #[allow(unused)], so you'll get unused variable warnings now!
