RFC: uniform API

The rfc API still used that "void* &"
switch to void** which makes more sense for the C interface.
Also, that's what we use on the RAW api

While we are at it, force the template to specify both
input *and* output typenames, so that it is easier
to switch between the linked library and the header-only

inline the template so that it can be reincluded
multiple times without redeinitions

Signed-off-by: Luca Fulchir <luker@fenrirproject.org>