unify all hashlike things under an abstract hash type

[StefanKarpinski]

This will allow them to all share a bunch of generic functionality. This includes HashTable, IdTable, and EnvHash. Could include more in the future.

[JeffBezanson]

What should it be called? Associative? AbstractTable?

[StefanKarpinski]

I think Associative is a good name.

[JeffBezanson]

Does this include sets?

[StefanKarpinski]

It could... we could treat sets as associative objects where the values are true or false?