Simple string interpolation.

This allows "%(...)" inside a string literal to interpolate the
stringified result of an expression.

It doesn't support custom interpolators or format strings, but we can
consider extending that later.

test/benchmark/fib.wren

@@ -9,4 +9,4 @@ var start = System.clock
 for (i in 1..5) {
   System.print(Fib.get(28))
 }
-System.print("elapsed: " + (System.clock - start).toString)
+System.print("elapsed: %(System.clock - start)")