fix bug in compilation of and' and or'; more robust underflow detection.

* module/system/il/compile.scm (codegen): Rewrite handling of `and' and
  `or' ghil compilation, because it was broken if drop was #t. Tricky
  bug, this one! Took me days to track down!

* module/system/repl/repl.scm: Export call-with-backtrace, which probably
  should go in some other file.

* src/vm.c (scm_vm_save_stack): Handle the fp==0 case for errors before
  we have a frame.

* src/vm_engine.h (NEW_FRAME, FREE_FRAME): Stricter underflow checking,
  raising the stack base to the return address, in an attempt to prevent
  inadvertant stack smashing (the symptom of the and/or miscompilation
  bug).
  (CHECK_IP): A check that the current IP is within the bounds of the
  current program. Not normally compiled in. Perhaps it should be?

* src/vm_system.c (halt): Set vp->ip to NULL. Paranoia, I know.
  (return): Call CHECK_IP(), if such a thing is compiled in.

* testsuite/Makefile.am (vm_test_files):
* testsuite/t-catch.scm:
* testsuite/t-map.scm:
* testsuite/t-or.scm: New tests.

testsuite/t-map.scm

@@ -0,0 +1,10 @@
+; Currently, map is a C function, so this is a way of testing that the
+; VM is reentrant.
+
+(begin
+
+  (define (square x)
+    (* x x))
+
+  (map (lambda (x) (square x))
+       '(1 2 3)))