TODO: CRAM split aux tags

[jkbonfield]

(A reminder ticket for myself so it hangs around like a bad smell and encourages me!)

Scramble diverged from samtools in how it encodes the aux tags. It now strictly creates one block per aux tag instead of merging them together loosely based on types (integer, string, etc). This normally improves compression slightly, although it isn't guaranteed.

I started porting the changes over, but it's very heavy on the hash table usage so it's a large task due to the radically different hash table implementations.