Handle recursion in Array#hash.

[jerith]

Three of the specs are still tagged. Two of these need Array#fill which isn't yet implemented and the third probably needs fixes to Hash#hash.

[alex]

I'm not sure I understand the logic here at all, at a minimum it needs an explanatory comment.

[alex]

Ok, now I understand it, using throw/catch seems way too complex, why not just use the normal recursion_guard and if the object is in a recursion return 0 or something.

[jerith]

That catches the recursion on the inside rather than the outside, and we've already modified the hash for the intervening layers. We get a.hash == [[a]].hash because of the XOR, but a.hash != [a].hash.

[jerith]

Turns out we need finer-grained recursion guarding before we can do this thing safely.

[jerith]

I'll update this to a simpler version that uses Thread.recursion_guard_outer once #460 is in.