[cgo] Pointer variables are not implemented properly.

Pointer variables are making the headers not compile. We tried to use interface{} style-pointers in order to abstract this for the user, but they seemed to crash.

My suggestion would be to handle pointers strictly as uint64 handles throughout all the go code.
@qmuntal: Do you have any objections?

Start by reproducing the header compilation failure described in the issue and trace how pointer variables are handled in the cgo-related Go code. Compare the current interface{}-style pointer handling with the proposed uint64-handle approach. Done means generated headers compile and pointer variables no longer crash, but the issue names no files or tests.

Written by the indexing model from the issue text.