Allow "*" on lists and strings to repeat them.

This is not implemented on Sequence because, at least for lists and
strings, I think users expect an eager result. Multiplying a string
should give you back a string, not a lazy sequence of repeated
characters.

This also mirrors "+" on strings and lists, which is eager. I like the
idea of having a general guideline that operators are eager.

Repetition is useful for arbitrary sequences, but for that maybe we
should add a "repeat()" method.

test/core/list/multiply.wren

@@ -0,0 +1,8 @@
+System.print([1, 2, 3] * 0) // expect: []
+System.print([1, 2, 3] * 1) // expect: [1, 2, 3]
+System.print([1, 2, 3] * 4) // expect: [1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3]
+
+// Doesn't modify original list.
+var a = [1, 2, 3]
+a * 5
+System.print(a) // expect: [1, 2, 3]

test/core/list/multiply_negative.wren

@@ -0,0 +1 @@
+[1, 2, 3] * -3 // expect runtime error: Count must be a non-negative integer.

test/core/list/multiply_not_int.wren

@@ -0,0 +1 @@
+[1, 2, 3] * 1.2 // expect runtime error: Count must be a non-negative integer.

test/core/list/multiply_not_num.wren

@@ -0,0 +1 @@
+[1, 2, 3] * "not num" // expect runtime error: Count must be a non-negative integer.

test/core/list/plus.wren

@@ -1,18 +1,11 @@
-var a = [1, 2, 3]
-var b = [4, 5, 6]
-var c = a + b
-var d = a + (4..6)
-var e = a + []
-var f = [] + a
-var g = [] + []
-
-System.print(a) // expect: [1, 2, 3]
-System.print(b) // expect: [4, 5, 6]
-System.print(c) // expect: [1, 2, 3, 4, 5, 6]
-System.print(d) // expect: [1, 2, 3, 4, 5, 6]
-System.print(e) // expect: [1, 2, 3]
-System.print(f) // expect: [1, 2, 3]
-System.print(g) // expect: []
+System.print([1, 2, 3] + [4, 5, 6]) // expect: [1, 2, 3, 4, 5, 6]
+System.print([1, 2, 3] + (4..6)) // expect: [1, 2, 3, 4, 5, 6]
+System.print([1, 2, 3] + "abc") // expect: [1, 2, 3, a, b, c]
+System.print([] + []) // expect: []
+System.print([1, 2] + []) // expect: [1, 2]
+System.print([] + [3, 4]) // expect: [3, 4]
 
 // Doesn't modify original list.
+var a = [1, 2, 3]
+a * 5
 System.print(a) // expect: [1, 2, 3]

test/core/list/plus_not_iterable.wren

@@ -0,0 +1 @@
+[1, 2, 3] + 4 // expect runtime error: Num does not implement 'iterate(_)'.