Completion by trait bounds for fnarg

[kngwyu]

By this PR, racer can complete method for generic type with trait bounds, like

fn f<T: Clone>(a: T) {
    let b = a.clo~
}

In addition, trait inheritance(like below) is also supported.

trait Tr: Clone {}
fn f<T: Tr>(a: T) {
    let b = a.clo~
}

This PR is related to #706, but what I implemented in this PR works only when FnArg has generic type T.
So, these cases are not supported now.

struct S<T> {
    mem: T
}
fn f<T: Clone>(a: S<T>) {
    let b = a.mem.clo~
}
impl<T: Clone> S<T> {
    fn f(self) {
        let b = a.mem.clo~
    } 
}

I'm going to implement these features after this PR and #825 are merged.

[kngwyu]

This change isn't compatible with current libsyntax so I close this.