Support for shortand &self in bridge

[LeonMatthesKDAB]

CXX supports using &self and self: Pin<&mut Self> instead of the longer: self: &T as long as there is only a single type in a given block.

This is very handy and we should support the same in CXX-Qt:

extern "C++Qt" {
    #[qobject]
    type MyObject = super::MyObjectRust;

    // No need to specify the type explicitly here, MyObject is the only type defined in this `extern` block, so `Self` refers to `MyObject` implicitly.
    fn my_function(&self) -> i32;
    fn my_other_function(self: Pin<&mut Self>) -> i32;
}

The open question we currently have is how this would deal with name resolution in the structuring phase...
So we may need to refactor our parser to return which objects/methods were defined in which block, instead of adding them all into a single structure like we currently do.