re-enable computed goto; fix ,help in the repl; subr dispatch optimizations

re-enable computed goto; fix ,help in the repl; subr dispatch optimizations

* m4/labels-as-values.m4: New file, checks for computed goto.

* configure.in: Use AC_C_LABELS_AS_VALUES.

* module/system/repl/command.scm (procedure-documentation): Extend the
  core's procedure-documentation in an ad-hoc way, so that ,help works.

* module/system/vm/core.scm (program-properties): New function.
  (program-documentation): New function.

* src/vm_engine.h (DROP, DROPN): Decrement sp before checking for
  underflow.

* src/vm_system.c (call, tail-call): Add some optimized dispatch for some
  C functions, so that we can avoid consing and the interpreter if
  possible. However currently it seems that I'm always getting the
  scm_call_* trampolines back.