Use pointer equality for instances `Eq (Array a)` and `Eq (SmallArray a)`?!

[sjakobi]

These instances are currently defined in terms of arrayLiftEq and smallArrayLiftEq.

Eq (PrimArray a) on the other hand makes use of sameByteArray.

[andrewthad]

I think it would be fine to add a fast path for the case when the two arrays are the same pointer.