improved HashMap.hashCode() implementation inspired by Clojure

[fmw]

There are two important changes:

• use bitwise XOR instead of plus operator to combine the hashcode of the key and value, which intertwines them and prevents collisions on hashmaps with values swapped between keys,
• cache the hash code internally, to avoid unnecessary recalculations.

I've also added a test that verifies the hashcode doesn't take the order in which the kv-pairs are supplied into account.

[fmw]

(the force push was to fix a typo in the test it description).

[emmanueltouzery]

just tiny things, but when we clear this up, no problem to merge this!

[emmanueltouzery]

thanks a lot! i'll try to release this ASAP