Make Sequence base class.

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

test/range/where.wren

@@ -0,0 +1,6 @@
+var a = 1..3
+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: []