Merge pull request #222 from bjorn/sequence_contains

Moved List.contains to Sequence.contains, added tests and docs

test/core/list/contains.wren

@@ -0,0 +1,6 @@
+var list = [1, 2, 3, 4, "foo"]
+
+IO.print(list.contains(2))      // expect: true
+IO.print(list.contains(5))      // expect: false
+IO.print(list.contains("foo"))  // expect: true
+IO.print(list.contains("bar"))  // expect: false