Reduce the die size by 8 bytes

[leethomason]

Description

The heavily used die is currently 96 bytes. With this PR, it is brought down to 88 bytes, by removing the _hash member. We still need the hash to store the die in the global map, but it can be computed on demand.

[fosterbrereton]

When the fatal attribute hashing PR lands, this one will be hard to merge. The problem is that deriving the hash requires the attributes of the die, which are no longer stored with the die in the FAH branch. We can still get this hash on an as-needed basis, but it will require going back to disk to re-read the die's attributes. We'll need to re-evaluate the tradeoffs here once the FAH branch lands.

[leethomason]

Good point; we'll hold off on this and look again after FAH.

[leethomason]

A wash to the FAH work.