Move let-expr resolvability check to Resolver
Use numerical indices in resolutions
Statically resolve global references
Capitalize stuff to match common C# style
Replace `with` keyword with `but`
Figure out multiple-binding let-expr semantics
Clean up visibility
Inject error handling into parser and scanner
Think about a way to hide record fields
	- Maybe a compile-time primitive for generating globally unique symbols?
		Would also need a syntax to use those symbols.
	- Use @ as an operator to convert a comptime-known string to an identifier

Do I need impredicativity or can I just use rank-2 polymorphism?
	- I think rank-2 is decidable.
	- If impredicativity, use bidirectional typechecking with Quick Look.
Equirecursive or isorecursive?