Wrap cgo calls with an interface

[qmuntal]

Changes:

• Abstract all cgo with an interface that accepts accepts and returns cgo types
• Do all the go<->cgo heavy lifting in Wrapper but delegate the real cgo call to the interface.
• Create one interface implementation for dynamic loading, maybe called DynamicWrapper. Put all the related c and go code in a separated files. Add an opt-out tag (act_dynload)? to this file so users that don't need it can remove it at build time.
• Create one interface implementation for static/dynamic linking, maybe called LinkWrapper. Put all the related c and go code in a separated files. Add an opt-in tag (act_link)? to this file for users who want to support this mode.

This allows to support the two scenarios and any combination of them with a small runtime overhead.

Updates: #123