add memoized expression accessors to scheme

add memoized expression accessors to scheme

* libguile/eval.c: Fix a comment.
  (eval): Abstract out the variable memoization into a function,
  scm_memoize_variable_access_x.

* libguile/memoize.c (memoized_tags): Fix a couple names.
  (scm_memoize_variable_access_x): New internal function. Actually it's
  public to Scheme, but we can't do much about that, because the new
  evaluator will need it.
  (scm_memoized_expression_typecode, scm_memoized_expression_data): New
  accessors for memoized code, for Scheme.
  (scm_memoized_typecode): Looks up the typecode for a symbol.