Improve function types

[dfellis]

One large issue with the current types is that functions are simply typed function, without their argument and return types declared.

The compiler will error out usage of higher-order functions that don't match what the higher-order function needs, but the error isn't super clear, nor is the source code, as you can't at-a-glance determine what it needs.

Creating a more advanced function-type syntax is absolutely necessary, above and beyond the type syntactic sugar mentioned in a previous issue.