gopy: consider support of a CFFI backend

[sbinet]

prompted by: https://www.reddit.com/r/Python/comments/3ig3ww/building_python_modules_with_go_15_xpost_from/cugc9ri

it could be interesting to have a CFFI backend for pypy's benefit.
maybe simpler to just tap into cppyy ?

see https://github.com/jbuberel/buildmodeshared/tree/master/gofrompython for a CFFI-based example

[corona10]

I am working on this issue.