Generalize fixed-length lists to listofs.

Closes PR10057.

collects/tests/typed-scheme/succeed/pr10057.rkt

@@ -0,0 +1,5 @@
+#lang typed-scheme
+(require scheme/match)
+(ann (match '(a b c)
+       [(list sym more ...) 1]
+       [else 1]) Integer)

collects/typed-scheme/types/convenience.rkt

@@ -31,6 +31,7 @@
       (match t*
         [(Value: '()) (-lst Univ)]
 	[(Value: 0) -Nat]
+        [(List: ts) (-lst (apply Un ts))]
         [(? (lambda (t) (subtype t -Nat))) -Nat]
         [(? (lambda (t) (subtype t -Integer))) -Integer]
         [(? (lambda (t) (subtype t -Flonum))) -Flonum]