RA22-015: Fix to_iterator's signature

Will be needed for an upcoming fix on call resolution. For the moment
to_iterator is unparsed as a regular function call in dsl_unparse, but
in the prelude it's a property, so unify towards it being a function.
Some discrepancies (see entity_iterator test) are ignored for the moment
due to a hole in type resolution.

contrib/lkt/language/prelude.lkt

@@ -35,7 +35,7 @@ trait Iterator {
 
 @builtin generic[T]
 struct Array implements Sized, Indexable[T], Iterator[T] {
-    @builtin @property fun to_iterator(): Iterator[T]
+    @builtin fun to_iterator(): Iterator[T]
 }
 
 @builtin generic[T]

testsuite/tests/contrib/lkt_semantic/builtin_methods/test.lkt

@@ -23,6 +23,6 @@ class Name : RootNode implements TokenNode {
     fun parent_nodes (): Array[RootNode] = node.parents()
     fun parent_nodes (): Array[RootNode] = node.parents(with_self=false)
 
-    fun node_iterator (): Iterator[RootNode] = node.parents().to_iterator
+    fun node_iterator (): Iterator[RootNode] = node.parents().to_iterator()
 
 }

testsuite/tests/contrib/lkt_semantic/builtin_methods/test.out

@@ -460,8 +460,11 @@ Id   <RefId "to_iterator" test.lkt:26:63-26:74>
      references <FunDecl prelude: "to_iterator">
 
 Expr <RefId "to_iterator" test.lkt:26:63-26:74>
-     has type <InstantiatedGenericType prelude: "Iterator[RootNode]">
+     has type <FunctionType prelude: "() -> Iterator[RootNode]">
 
 Expr <DotExpr test.lkt:26:48-26:74>
+     has type <FunctionType prelude: "() -> Iterator[RootNode]">
+
+Expr <CallExpr test.lkt:26:48-26:76>
      has type <InstantiatedGenericType prelude: "Iterator[RootNode]">