Type parameters not allowed on UFCS methods

[nikomatsakis]

The instantiate_path code incorrectly accounts for type parameter spaces, meaning that this program is rejected:

trait Foo<T> {
    fn get(&self) -> T;
}

impl Foo<i32> for i32 {
    fn get(&self) -> i32 { *self }
}

fn main() {
    let x: i32 = 1;
    Foo::<i32>::get(&x)
}

cc @huonw