Merge: lib/standard: optimize Array.to_s

Micro-optimized version of Array::to_s based on the results of benchmarks available in PR #685.

`./bin/nitg ./src/nitg.nit`
Before : `15.080 s`
After : `13.928 s`

Pull-Request: #686
Reviewed-by: Jean Privat <jean@pryen.org>

diff --cc lib/standard/collection/array.nit
Simple merge

diff --cc tests/sav/test_new_native_alt1.res
index 0d3a41d,7b6ea2d..83c6493
--- 1/tests/sav/test_new_native_alt1.res
--- 2/tests/sav/test_new_native_alt1.res
+++ b/tests/sav/test_new_native_alt1.res
@@@ -1,4 -1,4 +1,4 @@@
- Runtime error: Cast failed. Expected `NativeArray#0`, got `Bool` (../lib/standard/collection/array.nit:753)
 -Runtime error: Cast failed. Expected `NativeArray#0`, got `Bool` (../lib/standard/collection/array.nit:742)
++Runtime error: Cast failed. Expected `NativeArray#0`, got `Bool` (../lib/standard/collection/array.nit:755)
  NativeString
  N
  Nit