Make Sequence base class.

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

test/list/map.wren

@@ -1,5 +1,3 @@
 var a = [1, 2, 3]
-var inc = fn (x) { return x + 1 }
-var b = a.map(inc)
-
+var b = a.map(fn (x) x + 1)
 IO.print(b) // expect: [2, 3, 4]

test/list/type.wren

@@ -1,4 +1,5 @@
 IO.print([] is List)      // expect: true
+IO.print([] is Sequence)  // expect: true
 IO.print([] is Object)    // expect: true
 IO.print([] is Bool)      // expect: false
 IO.print([].type == List) // expect: true

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: []