will_return_by_value() does not need second (size) parameter

[thoni56]

Since that constraint is intended for returning structs by value, we should (at some point) change from

will_return_by_value(<struct>, sizeof(<type of struct>));

to just

will_return_by_value(<struct>);

since the macro itself can do the sizeof and the C compiler will always know that.

It is an incompatible change, but if I knew how to work with argument counting macro magic (like @matthargett does ;-) then we could probably make some kind of overloading allowing us to move to it without breaking backwards compatibility.