Revisit non-primitive return type in callback

[mara004]

In retrospect, I think remapping a non-primitive pointer type to c_void_p, as the UNCHECKED() wrapper did, actually seems right. I probably was subject to a misunderstanding back then.

However, it would be great if we could do this directly at bindings construction time, rather than through a runtime wrapper in the bindings.

Also, a real-world example would be great for testing.

[mara004]

Note, historically, the UNCHECKED() wrapper caused a bug during the python 2 -> 3 migration: ctypesgen#70 ctypesgen#73

That's not relevant anymore, but it hints that templates tend to be wonky, which is part of the reason why I'd prefer to avoid them...

[mara004]

See also

1. https://stackoverflow.com/questions/33005127/python-ctypes-callback-function-gives-typeerror-invalid-result-type-for-callba
2. https://stackoverflow.com/questions/50035173/return-type-in-case-of-callback-functions

Esp. Eryk Sun's comments on (1).

[mara004]

OK, I think I added back a corrected verison of the UNCHECKED wrapper.
See ctypesgen#70 (comment) for what I changed.