Fix MSVC 10.0 error

Apparently it does not support local typedefs.
Revert "Cache discovered callback addresses"

This reverts commit ce8bff16edab5d8184c08edc552a4d3d8dc98ca3.

Conflicts:
	src/core.c
	src/internal/callback.c
	src/internal/callback.h
	src/internal/plugin.c
	src/internal/plugin.h

That's now worth it really.