Resolve #17

This patch (re)installs the check for continuation type
well-formedness.

Currently, it allows only continuation types to be indexed by plain
function types.

interpreter/valid/valid.ml

@@ -183,7 +183,9 @@ let check_func_type (c : context) (ft : func_type) at =
 
 let check_cont_type (c : context) (ct : cont_type) at =
   match ct with
-  | ContT ft -> check_heap_type c ft at
+  | ContT (VarHT (StatX x)) ->
+     let _dt = func_type c (x @@ at) in ()
+  | _ -> error at "ill-formed continuation type"
 
 let check_table_type (c : context) (tt : table_type) at =
   let TableT (lim, t) = tt in

test/core/cont.wast

@@ -205,6 +205,11 @@
       (drop)))
   "non-continuation type 0")
 
+(assert_invalid
+  (module
+    (type $ct (cont $ct)))
+  "non-function type 0")
+
 ;; Simple state example
 
 (module $state