Consolidate `Param`/`Ref`/`InterfaceRef` etc

[kennykerr]

A number of conversion types and traits have been developed to solve different problems. Some like Param are used by windows-bindgen while others are more suitable for hand-authored APIs. It's just a bit confusing and hard to know what the right tool is where there are so many options.

[kennykerr]

We should try and use Ref for interface traits and delegate closures. Its way simpler than the use of Option<&T> or &Option<T>.

Particularly, the difference in the closure signature between TypedEventHandler<T, A> and EventHandler<T> is quite confusing.

[kennykerr]

Ideally we could have an AsRef that worked something like this:

pub trait AsRef<T> where T: Type<T> {
    fn as_ref<'a>(&'a self) -> Ref<'a, T>;
}

impl<T: Type<T>> AsRef<T> for T {
    fn as_ref<'a>(&'a self) -> Ref<'a, T> {
        unsafe { transmute_copy(self) }
    }
}

impl<'a, T: Type<T>> AsRef<T> for &'a T {
    fn as_ref(&self) -> Ref<'a, T> {
        unsafe { transmute_copy(self) }
    }
}

impl<'a, T: Type<T>> AsRef<T> for Option<&'a T> {
    fn as_ref(&self) -> Ref<'a, T> {
        unsafe {
            match self {
                Some(value) => transmute_copy(value),
                None => Ref::zeroed(),
            }
        }
    }
}

This would then provide more uniform parameter binding across regular and generic parameters and be far simpler in general.

As noted in #3394 this doesn't support polymorphism the way that Param does though.