Add support for tuple patterns

[atomb]

Right now, the only way to extract elements from a tuple is with positional selectors (x.0, etc.). It would be nice to allow pattern matching syntax, too, so you could do, for instance:

let f (x, y) = ...

or

(a, b) <- ...

and so on.

[brianhuffman]

Commit 46c4fc4 adds support for the forms of tuple binders mentioned above, as well as lambda binders like \(x, y) -> e. Support for declarations with pattern bindings like let (a, b) = ... is still missing, though. I'll close the ticket once I've implemented that as well.