Support impl for type parameter `impl<T> Trait for T`

Currently we don’t look for impls like impl<T> Trait for T, so this test fails

trait Trait {
    fn foo(&self) {}
}    //X
impl<T> Trait for T {}
struct X;
fn main() {
    X.foo();
}   //^

It is pretty easy to implement. Here we indexing the impl by the type. We should check if type is a type parameter of this impl (go to parent psi, find type parameters declaration and check the type name is the same), then if it is a type parameter, save it to index with a specific key that is invalid identifier (like #). Then while finding impls retreive index twice: with actual type name and with our specific key.

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[Undin]

I’ve previously thought about it I want to grab it:)

[vlad20012]

@Undin I don’t see a much difference between impl<T: Foo> for T and impl<T: Foo> for S<T> in terms of trait boud matching. We do not support both =) So it’s another issue. (see RsPreciseTraitMatchingTest)

@farodin91 no-no-no, #1649 is another problem