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/delta_blue.wren

@@ -696,4 +696,4 @@ for (i in 0...40) {
 }
 
 System.print(total)
-System.print("elapsed: " + (System.clock - start).toString)
+System.print("elapsed: %(System.clock - start)")