look up scheme translators by value, not by name

look up scheme translators by value, not by name

* module/language/scheme/translate.scm (custom-transformer-table): Rename
  from `primitive-syntax-table', because now it will handle procedural
  values as well.
  (lookup-transformer): Update for renaming. Look up custom transformers
  by value, not name.
  (make-pmatch-transformers): Key the transformer table by value, not
  name.