fix compilation of quasiquote with splicing and improper lists

* libguile/vm-engine.h (POP_CONS_MARK): New macro, analagous to
  POP_LIST_MARK; used in quasiquote on improper lists.

* libguile/vm-i-system.c (cons-mark): New instruction. You know the
  drill, remove all your .go files please.

* module/system/il/compile.scm (codegen): Compile quasiquoted improper
  lists with splices correctly. Additionally check that we don't have
  slices in the CDR of an improper list.

* testsuite/t-quasiquote.scm: Add a test for unquote-splicing in improper
  lists.

libguile/vm-engine.h

@@ -356,6 +356,19 @@ do {						\
   PUSH (l);					\
 } while (0)
 
+#define POP_CONS_MARK()				\
+do {						\
+  SCM o, l;					\
+  POP (l);                                      \
+  POP (o);					\
+  while (!SCM_UNBNDP (o))			\
+    {						\
+      CONS (l, o, l);				\
+      POP (o);					\
+    }						\
+  PUSH (l);					\
+} while (0)
+
 
 /*
  * Instruction operation