Make Sequence base class.

This lets us share functionality between List and Range (and
other user-defined sequence types).

test/list/where.wren

@@ -1,8 +1,6 @@
 var a = [1, 2, 3]
-var moreThan1 = fn (x) { return x > 1 }
-var moreThan10 = fn (x) { return x > 10 }
-var b = a.where(moreThan1)
-var c = a.where(moreThan10)
-
+var b = a.where(fn (x) x > 1)
 IO.print(b) // expect: [2, 3]
+
+var c = a.where(fn (x) x > 10)
 IO.print(c) // expect: []