Add easier support for ObjC blocks in p/invokes

[spouliot]

It's a lot of boilerplate code that's easy to get wrong (at compile or run time).

Either

• have sharpie generate the required code;
• have the generator (who already does it for ObjC) spit out the internal code needed based on a signature

I think I prefer the later... since the code already exists there (and any change would need to be done only once).

[rolfbjarne]

See also #4543

[rolfbjarne]

Duplicate of #15783.