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/method_call.wren

@@ -65,4 +65,4 @@ for (i in 0...n) {
 }
 
 System.print(ntoggle.value)
-System.print("elapsed: " + (System.clock - start).toString)
+System.print("elapsed: %(System.clock - start)")