fix compilation of quasiquote with splicing and improper lists

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.