Merge: Improve hash perfs
Some improvements.
## nitg nitg.nit
before: 0m9.688s
after: 0m9.172s
## ./nit naive_interpreter.nit
usage of HashCollection before:
average length: 128.41
average capacity: 294.06 (229.01%)
average number of collisions: 113571 (35.27%)
average length of collision: 3.74
usage of HashCollection after:
average length: 128.40
average capacity: 327.42 (255.00%)
average number of collisions: 62608 (19.44%)
average length of collision: 2.29
All this is needed for the ai library
Pull-Request: #651
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>