Add constraints to typechecker, fix old tests, add new ones

priv/stdlib/List.aes

@@ -29,9 +29,11 @@ namespace List =
     []   => abort("drop_last_unsafe: list empty")
 
 
-  function contains(e : 'a, l : list('a)) = switch(l)
-    []   => false
-    h::t => h == e || contains(e, t)
+  function
+    contains : 'a is eq; ('a, list('a)) => bool
+    contains(e, l) = switch(l)
+      []   => false
+      h::t => h == e || contains(e, t)
 
 /** Finds first element of `l` fulfilling predicate `p` as `Some` or `None`
  * if no such element exists.

priv/stdlib/Option.aes

@@ -30,7 +30,9 @@ namespace Option =
     None    => abort(err)
     Some(x) => x
 
-  function contains(e : 'a, o : option('a)) = o == Some(e)
+  function
+    contains : 'a is eq; ('a, option('a)) => bool
+    contains(e, o) = o == Some(e)
 
   function on_elem(o : option('a), f : 'a => unit) : unit = match((), f, o)
 

priv/stdlib/String.aes

@@ -90,6 +90,7 @@ namespace String =
           Some(ix)
 
   private function
+    is_prefix : (list(char), list(char)) => option(list(char))
     is_prefix([], ys) = Some(ys)
     is_prefix(_, [])  = None
     is_prefix(x :: xs, y :: ys) =