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>

tests/gpgscm/scheme.h

@@ -90,6 +90,11 @@ extern "C" {
 # define USE_COMPILE_HOOK 1
 #endif
 
+/* Enable faster opcode dispatch.  */
+#ifndef USE_THREADED_CODE
+# define USE_THREADED_CODE 1
+#endif
+
 #ifndef USE_STRCASECMP   /* stricmp for Unix */
 # define USE_STRCASECMP 0
 #endif