Incorrect fn subtyping with bound regions and 'static

[nikomatsakis]

This test file incorrect returns a compilation error:

struct S;
type F = fn<'cx>(&'cx S) -> &'cx S;
fn foo(x: &S) -> &'static S { fail!() }
fn want_F(f: F) { }
pub fn main() {
    want_F(foo); // Incorrect error here
}

In particular, a fn like foo which always returns a pointer with 'static lifetime should be considered a subtype of a fn type that always returns pointer with same lifetime as its input.