Check that the self type in an extern spec for a trait impl matches the external definition.

[nilehmann]

We currently accept nonsensical extern specs on trait implementations like this one

use flux_rs::*;

#[extern_spec(std::ops)]
impl<A: Step> Iterator for Range<usize> {}

Given the way we expand the macro, A won't be reported as non-constrained, and Range<usize> passes further validations because it is less generic than the extern implementation so it still implements Iterator. We should check that the self-type of extern specs for trait implementations matches the external definition.