Fix Iter.take to handle infinite iterator

Previously, the `take` method on an infinite stream, such as `Rand`,
would cause infinite recursion.

packages/itertools/_test.pony

@@ -421,6 +421,15 @@ class iso _TestIterTake is UnitTest
 
     h.assert_array_eq[String](expected, actual)
 
+    let infinite =
+      Iter[U64](
+        object ref
+          fun ref has_next(): Bool => true
+          fun ref next(): U64 => 0
+        end)
+
+    h.assert_eq[USize](3, infinite.take(3).collect(Array[U64]).size())
+
 class iso _TestIterTakeWhile is UnitTest
   fun name(): String => "itertools/Iter.take_while"
 

packages/itertools/iter.pony

@@ -678,16 +678,19 @@ class Iter[A] is Iterator[A]
     ```
     `1 2 3`
     """
-    filter_stateful(
+    Iter[A](
       object
         var _countdown: USize = n
 
-        fun ref apply(a: A!): Bool =>
+        fun ref has_next(): Bool =>
+          _countdown > 0
+
+        fun ref next(): A ? =>
           if _countdown > 0 then
             _countdown = _countdown - 1
-            true
+            _iter.next()?
           else
-            false
+            error
           end
       end)