Make (Set a) a subtype of (Sequenceof a).

Closes PR13485.
Closes PR12615.

original commit: 4f9f00680ee560ee59cddd5bdb3fb737c6bcc7f1

collects/tests/typed-racket/succeed/for-set.rkt

@@ -0,0 +1,3 @@
+#lang typed/racket
+(for ([i (set 1 2 3)])
+  (displayln i))

collects/tests/typed-racket/unit-tests/subtype-tests.rkt

@@ -78,6 +78,7 @@
 
    [(-val 6) -Number]
    [(-val 'hello) -Symbol]
+   [(-set -Number) (make-Sequence (list -Number))]
    [((Un -Symbol -Number) . -> . -Number) (-> -Number -Number)]
    [(-poly (t) (-> -Number t)) (-mu t (-> -Number t))]
    ;; not subtypes

collects/typed-racket/infer/infer-unit.rkt

@@ -474,6 +474,8 @@
            (cg t t*)]
           [((Hashtable: k v) (Sequence: (list k* v*)))
            (cgen/list V X Y (list k v) (list k* v*))]
+          [((Set: t) (Sequence: (list t*)))
+           (cg t t*)]
           ;; ListDots can be below a Listof
           ;; must be above mu unfolding
           [((ListDots: s-dty dbound) (Listof: t-elem))

collects/typed-racket/types/subtype.rkt

@@ -299,6 +299,8 @@
                (subtype* A0 -Nat t*)]
               [((Hashtable: k v) (Sequence: (list k* v*)))
                (subtypes* A0 (list k v) (list k* v*))]
+              [((Set: t) (Sequence: (list t*)))
+               (subtype* A0 t t*)]
               ;; special-case for case-lambda/union with only one argument              
               [((Function: arr1) (Function: (list arr2)))
                (when (null? arr1) (fail! s t))