Borrow good ideas from moniker

[lambdageek]

moniker is a binding combinators library for Rust that takes its inspiration from Unbound.

https://github.com/brendanzab/moniker

One interesting design decision was to split up Alpha into a pattern type class and a term type class.

The main change that we make is to have two separate traits (BoundTerm and BoundPattern) in place of Unbound's single Alpha type class. We've found that this better captures the semantics of the library, and greatly cuts down on the potential for accidental misuse.

It's worth exploring whether this makes sense for us, too.

[TOTBWF]

In a similar vein Ignore<T> looks very useful. #30 should be a starting point for an implementation.