Type Extractors

[baetheus]

Almost every exported Kind and Type in fun could use an extraction type. The goal of this issue is:

1. Decide on a cohesive naming design for the type level extractors.
2. Implement type extractors for all exported types that can use it.

A type extractor looks like the following:

type MyArray<A> = readonly A[];

type ExtractMyArray<T> = T extends MyArray<infer A> ? A : never;