Merge: lib/string: `Int::to_s` shortcuts 0 and 1

Valgrid said it is used a lot.

So I mixed (`-m`) nitc with the following module

~~~nit
import counter

redef class Int
	redef fun to_s
	do
		sys.itos_cpt.inc(self)
		return super
	end
end

redef class Sys
	var itos_cpt = new Counter[Int]
	redef fun run
	do
		super
		itos_cpt.print_summary
		itos_cpt.print_elements(10)
	end
end
~~~

The result shows that `0` and `1` are the top `to_s`-ized numbers.

~~~
  0: 13554 (9.29%)
  1: 10012 (6.86%)
  2: 5671 (3.88%)
~~~

So I just shortcut-them to reduce allocations.

With nitc/nitc/nitc:
before: 0m6.756s
after: 0m6.632s (-2%)

Pull-Request: #1207
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>