Also cuts down on the hoops we have to jump through from msg_send to reach the actual message sending

In Objective-C, having a null pointer receiver is valid, but with `msg_send!` it would be converted into a reference, which is undefined behavior. The reference was converted into a pointer immediately after, but it is still UB. There might also be a mutable reference somewhere else in the program, which would now be aliased.

With this, we can now stop de-referencing the receiver of msg_send!, and still have Id work ergonomically. However, null pointer receivers are still discouraged because of all the other issues they cause!

This change additionally allows sending messages to NonNull<T>, Option<&[mut] T> and Option<NonNull<T>>.

This makes the syntax match Rust expectations a bit closer IMO

Rustfmt can't do this for us

We actually don't want to encourage sending messages to nil objects, since that is only supported for selectors that return pointers