Generate trampoline and registrar tests that tests if a return type requires objc_msgSend or objc_msgSend_stret.
Now it's much easier to test new return types (a single line of code), which
avoids a _lot_ of copy-pasting, and makes sure all the different variations
are tested properly.
These new tests found several bugs, which are fixed in subsequent commits.
Support binding NSObjects as IntPtr. This is usually not
a problem, because when we fetch the ObjC signature for a
method, we usually get the signature as exported by us,
(in which case a parameter bound as 'IntPtr' would be treated
as 'void *' in the dynamic registrar) *except* when the
selector corresponds with a protocol the type implements,
in which case we get the signature as defined in the protocol.
https://bugzilla.xamarin.com/show_bug.cgi?id=41132