Support for variable-arity generics

[sstrickl]

While we can do Enumerators over a single type initially to get support rolling, I imagine that for Enumerators (and things like Tuples), in the long run we eventually want support for something akin to Variable-Arity Generic Interfaces.

There's also Practical Variable-Arity Polymorphism, which would be useful if there are any block-taking methods that work like Scheme's map. (That is, you take a block and an arbitrary number of collections, where the arity of the block is related to the number of collections and the type of each block argument is the same as the corresponding collection element type. The latter paper shows off such examples.)

[jtoman]

This seems to be a duplicate of #12. Can you merge the above description into that ticket?

[sstrickl]

Sorry, will do!