Implement `Eq`

[414owen]

RoaringBitmap has a PartialEq instance, but no Eq instance.
PartialEq doesn't guarantee reflexivity.
The PartialEq here is your standard, derived, structural equality, until we reach the Store type, where we do this, comparing equivalent storage types piecewise.

Judging by the spec, containers with equal elements should also be stored in the same type of backing store, an array for up to 4096 ints, and a 2^16 bit bitmap otherwise.

This should, if I'm reading this right, mean PartialEq is in fact reflexive.
This means an Eq instance would make sense.