complete default methods for traits

[lkuper]

Implement the first two parts of this proposal (the third part, dealing with instance coherence, is orthogonal).

[lkuper]

fc9c4c3 is the beginning of support for default methods in traits.

[lkuper]

Default methods get through typeck as of 293f371.

[lkuper]

Per IRC discussions, it seems that a big part of the remaining work (possibly the big part) is monomorphization. This is now @msullivan's bug, since I'm back at school and, alas, unlikely to work on it any time soon, and he's better equipped to deal with it than I am. :)