Add reduce function on Sequence

test/list/reduce.wren

@@ -0,0 +1,9 @@
+var a = [1, 4, 2, 1, 5]
+var max = new Fn {|a, b| a > b ? a : b }
+var sum = new Fn {|a, b| a + b }
+
+IO.print(a.reduce(max)) // expect: 5
+IO.print(a.reduce(10, max)) // expect: 10
+
+IO.print(a.reduce(sum)) // expect: 13
+IO.print(a.reduce(-1, sum)) // expect: 12

test/list/reduce_single_item.wren

@@ -0,0 +1 @@
+[1].reduce {|a, b| a } // expect runtime error: Range start out of bounds.

test/list/reduce_wrong_arity.wren

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+[1, 2, 3].reduce {|x, y, z| x } // expect runtime error: Function expects more arguments.