lib/standard/string: Array.to_s micro-optimization

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`

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>