Don't produce extra errors for top-level defines.

Lots more error message improvements for application.
Work on polydots apply.
Extend environment only in the proper place in plambda.
Don't let a variable unify with a dotted variable.

original commit: c8a2810742514c9f78b006930e6f327794af0228

collects/typed-scheme/private/type-utils.ss

@@ -22,7 +22,8 @@
          tc-result-t
          unfold
          (struct-out Dotted)
-         (struct-out DottedBoth))
+         (struct-out DottedBoth)
+         just-Dotted?)
 
 
 ;; substitute : Type Name Type -> Type
@@ -134,4 +135,8 @@
 
 ;; t is (make-F v)
 (define-struct Dotted (t))
-(define-struct (DottedBoth Dotted) ())
+(define-struct (DottedBoth Dotted) ())
+
+(define (just-Dotted? S)
+    (and (Dotted? S)
+         (not (DottedBoth? S))))