Allow impl item from anywhere

[udoprog]

Like the title says, support adding an impl <type> block anywhere and have it being handled correctly.

Like:

struct Foo;

pub fn main() {
    let value = {
        impl Foo {
            fn lol(self) {
                4
            }
        }
    };

    assert_eq!(Foo.lol(), 4);
}

This required the introduction of an intermediary step during indexing. We collect all impl blocks, and once the indexing queue is empty we start processing them. This ensures that imports are visible to the impl blocks and any other items (local or otherwise) which might affect the impl target. Luckily anything inside of the impl block itself can never be used as a target.