gpgscm: Reduce opcode dispatch overhead.

* tests/gpgscm/scheme.c (s_thread_to): New macro.
(CASE): Likewise.
(opexe_[0-6]): Use 'CASE' instead of 'case' statements, replace
's_goto' with 's_thread_to' where applicable.

This is a straight-forward optimization that replaces 's_goto' in
certain cases. Instead of returning to the calling function, and
dispatching the next opcode, we can jump to the opcode handler.

• Signed-off-by: Justus Winter <justus@g10code.com>