Move codePointAt() to separate CodePointSequence class.

builtin/core.wren

@@ -130,6 +130,7 @@ class WhereSequence is Sequence {
 
 class String is Sequence {
   bytes { StringByteSequence.new(this) }
+  codePoints { StringCodePointSequence.new(this) }
 }
 
 class StringByteSequence is Sequence {
@@ -144,6 +145,18 @@ class StringByteSequence is Sequence {
   count { _string.byteCount_ }
 }
 
+class StringCodePointSequence is Sequence {
+  construct new(string) {
+    _string = string
+  }
+
+  [index] { _string.codePointAt_(index) }
+  iterate(iterator) { _string.iterate(iterator) }
+  iteratorValue(iterator) { _string.codePointAt_(iterator) }
+
+  count { _string.count }
+}
+
 class List is Sequence {
   addAll(other) {
     for (element in other) {