The cgo pointer passing rules forbid passing a Go pointer to C if that
pointer points to memory containing other Go pointers. This is true
even if the Go pointer is converted to uintptr.
This change fixes the code to use a handle instead, and to look up the
handle in the callback function.
A call now doesn't have to do any reflection, it just blindly invokes
a bunch of argument and return value handlers to execute the translation,
and the safety of the translation is determined at registration time.
Yasuhiro Matsumoto