add "memoizer" data type

add "memoizer" data type

* libguile/memoize.h:
* libguile/memoize.c (MAKMEMO_APPLY): Take the proc and args separately.
  (scm_tc16_memoizer, SCM_MEMOIZER_P, SCM_MEMOIZER): New smob type, for
  "syntax" like @apply that have normal evaluation order.
  (memoize_env_ref_macro, memoize): Refactor to handle "memoizers"
  generically.

  Reorganize the list of SCM_SYNTAX forms.

  Move @apply, @dynamic-wind, @call-with-current-continuation,
  @call-with-values, @prompt handling down to be generic "memoizer" data
  types.

  (scm_memoizer_p, scm_memoizer): New functions, exposed to Scheme. Will
  be used by psyntax.