Abstract List's toString method to a more general join method on Sequence.

test/list/join.wren

@@ -0,0 +1,23 @@
+// Handle empty list.
+IO.print([].join(",") == "") // expect: true
+
+// Handle a simple list with an empty delimeter.
+IO.print([1, 2, 3].join("")) // expect: 123
+
+// Handle a simple list with no separator.
+IO.print([1, 2, 3].join) // expect: 123
+
+// Does not quote strings.
+IO.print([1, "2", true].join(",")) // expect: 1,2,true
+
+// Nested lists.
+IO.print([1, [2, [3], 4], 5].join(",")) // expect: 1,[2, [3], 4],5
+
+// Calls toString on elements.
+class Foo {
+  toString { "Foo.toString" }
+}
+
+IO.print([1, new Foo, 2].join(", ")) // expect: 1, Foo.toString, 2
+
+// TODO: Handle lists that contain themselves.

test/list/join_sep_non_string.wren

@@ -0,0 +1 @@
+[1, 2, 3].join(2) // expect runtime error: Right operand must be a string.