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