Prevent -let-internal name from leaking to users

Because -let-internal wasn't rename-out'd, users would see its name in
syntax error messages. Adding #:context forces the error messages to be
phrased in terms of the given form.

typed-racket-lib/typed-racket/base-env/prims.rkt

@@ -201,6 +201,7 @@ the typed racket language.
   (let ([mk (lambda (form)
               (lambda (stx)
                 (syntax-parse stx
+                  #:context form
                   [(_ (bs:optionally-annotated-binding ...) . body)
                    (quasisyntax/loc stx (#,form (bs.binding ...) . body))])))])
     (values (mk #'let) (mk #'let*) (mk #'letrec))))

typed-racket-test/fail/let-internal-nameleak.rkt

@@ -0,0 +1,4 @@
+#;
+(exn-pred exn:fail:syntax? #rx"let: expected identifier")
+#lang typed/racket
+(let ([() 5]) (void))