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/string/multiply.wren

@@ -0,0 +1,3 @@
+System.print("|" + "abc" * 0 + "|") // expect: ||
+System.print("abc" * 1) // expect: abc
+System.print("abc" * 4) // expect: abcabcabcabc

test/core/string/multiply_negative.wren

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

test/core/string/multiply_not_int.wren

@@ -0,0 +1 @@
+"abc" * 1.2 // expect runtime error: Count must be a non-negative integer.

test/core/string/multiply_not_num.wren

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