Typeclass-based extractors

[propensive]

The implementation of the n extractor in Nomenclature constructs an object with both apply and unapply methods. This approach could be generalized to implement any extractor, with apply and unapply being implemented with typeclasses.

The type of the interpolation object would be something like, Interpolation[("before", "after"), "prefix"] for an application of prefix"before${x}after" or case prefix"before${x}after".

This wouldn't change much for the implementation of interpolation expressions, but would make pattern matches clearer.