Association protocol

Daniel Ehrenberg implemented a generic protocol for associative data structures, much like the sequence protocol we have already. Hashtables, association lists and binary search trees (from libs/) implement this protocol now. Dan wrote an article about porting existing code that uses the old hashtable-specific words (which are gone) to the association protocol.

On a related note, lately more contributors have been venturing into the core, and not just working on stuff in libs/ and apps/. This is a good thing, in my opinion. I don't want the core language to be a mysterious, dark thing, and I'd love it if people learned enough about how the compiler works to contribute new optimizations, etc.