vinyl: remove engine wrapper functions

The engine infrastructure was initially implemented in C++ so we
needed the wrappers to provide C++ API to Vinyl. Now everything is
in C so we don't need them any more. Let's fold them in vinyl.c.

Note, this patch does not touch vinyl_engine, vinyl_index, and
vinyl_iterator structures, they are still there, it just gets rid
of the intermediate layer of wrapper functions, which is not needed
any more.