clean up string-constants expansion and add dynamic-string-constant[s]

This saves about 200 bytes per use of (string-constant ...) in the .zo file.

Also, it now only requires a single string-constants file in phase 1
(instead of all 13) so that should be a savings of memory and time
when compiling .zos, too.

original commit: 2ac73eee39e1b98a1d3d83cd3ed566d578625e4f

pkgs/typed-racket-pkgs/typed-racket-lib/typed-racket/base-env/base-special-env.rkt

@@ -30,9 +30,8 @@
   ;; make-promise
   [(make-template-identifier 'delay 'racket/private/promise)
    (-poly (a) (-> (-> a) (-Promise a)))]
-  ;; language
-  [(make-template-identifier 'language 'string-constants/string-constant)
-   -Symbol]
+  [(make-template-identifier 'dynamic-string-constant 'string-constants/string-constant)
+   (-> -Symbol -String)]
   ;; qq-append
  [(make-template-identifier 'qq-append 'racket/private/qq-and-or)
   (-poly (a b)