Rust: Fix context-based type inference explosion

[hvitved]

The following simple program would make our type inference implementation explode:

#[derive(Default)]
struct S;

fn f() -> S {
    let x = Default::default();
    From::from(x)
}

Firstly, because of a getSuccessor vs getAssocItem bug, From::from would allow for any sub trait of From, for example the Int trait, which meant that x could be inferred to have type bool. Secondly, since many from implementations are polymorphic in their argument, we should really be checking that the return type matches the type of the calling context.

This PR makes those two adjustments, and adds the regression test.