Added Sequence.take and Sequence.skip

These lazy iterator producing methods are useful when working with
arbitrary sequences and you need to skip or take some number of elements
at the start.

test/core/sequence/skip.wren

@@ -0,0 +1,28 @@
+class TestSequence is Sequence {
+  construct new() {}
+
+  iterate(iterator) {
+    if (iterator == null) return 1
+    if (iterator == 3) return false
+    return iterator + 1
+  }
+
+  iteratorValue(iterator) { iterator }
+}
+
+var test = TestSequence.new().skip(0)
+
+System.print(test is Sequence) // expect: true
+System.print(test) // expect: instance of SkipSequence
+
+// Skipping 0 changes nothing
+System.print(test.toList) // expect: [1, 2, 3]
+
+// Skipping 1 works
+System.print(test.skip(1).toList) // expect: [2, 3]
+
+// Skipping more than length of sequence produces empty list
+System.print(test.skip(4).isEmpty) // expect: true
+
+// Skipping less than 0 changes nothing
+System.print(test.skip(-10).toList) // expect: [1, 2, 3]

test/core/sequence/take.wren

@@ -0,0 +1,28 @@
+class TestSequence is Sequence {
+  construct new() {}
+
+  iterate(iterator) {
+    if (iterator == null) return 1
+    if (iterator == 3) return false
+    return iterator + 1
+  }
+
+  iteratorValue(iterator) { iterator }
+}
+
+var test = TestSequence.new().take(3)
+
+System.print(test is Sequence) // expect: true
+System.print(test) // expect: instance of TakeSequence
+
+// Taking 0 produces empty list
+System.print(test.take(0).isEmpty) // expect: true
+
+// Taking 1 works
+System.print(test.take(1).toList) // expect: [1]
+
+// Taking more than length of sequence produces whole sequence
+System.print(test.take(4).toList) // expect: [1, 2, 3]
+
+// Taking less than 0 produces empty list
+System.print(test.take(-10).isEmpty) // expect: true