Include new instruction execute_cpred to perform tail optimisation for

builtins. Required changes:
- be careful about creeping in deallocate: it may be followed by
something that is not a cut nor a proceed.
- include new instruction in absmi.c: it is a merge of execute and
call_cpred.
- change compiler to generate execute even for C builtins.
- be careful with dexecute: it may not be done if execute_op is a C
builtin.
- if we are in execute_cpred, the garbage collector cannot trust P:
instead it must look at CP to find out the size of the current
environment. The macro gc_P receives that information.
- We don't need to change CP if we do a meta-call from within
execute_cpred (and we in fact cannot). Check places where we do
meta-calls: exec, clause in cdmgr, and lu_recorded.

C/readutil.c

@@ -124,7 +124,7 @@ p_stream_to_codes(void)
       RESET_VARIABLE(h0);
       ARG4 = AbsPair(HBASE);
       ARG5 = (CELL)h0;
-      if (!Yap_gcl((ASP-HBASE)*sizeof(CELL), 5, ENV, P)) {
+      if (!Yap_gcl((ASP-HBASE)*sizeof(CELL), 5, ENV, gc_P(P,CP))) {
 	Yap_Error(OUT_OF_STACK_ERROR, ARG1, "read_stream_to_codes/3");
 	return FALSE;
       }