Call default methods on bound typarams more correctly

[brson]

r? @pcwalton

This makes test cases like the following work:

trait A {
    fn g() -> int { 10 }
}

impl int: A { }

fn f<T: A>(i: T) {
    assert i.g() == 10;
}

fn main () {
    f(0);
}

What doesn't work is if trait A is itself parameterized. This commit includes such testcases, xfailed. As with each of my commits to traits, the hacks involved make me feel uncomfortable. Trans isn't told specifically that it's implementing a default method - instead method lookup on the impl fails so trans decides that must be what's going on then starts grovelling for default implementations.

[brson]

This also doesn't handle the cross-crate case in trans.

[pcwalton]

r=me