Interior mutability

[michaelsproul]

This is an attempt to remove the need for mutability in TreeHash, by using ArcSwap and RwLock to handle the pending updates.

Alternative to these other 2 approaches which hit dead ends:

• Change trait to take self by value tree_hash#23
• Add TreeHashMut tree_hash#24

[dapplion]

What's the expected overhead of these locks?

[michaelsproul]

A few ms max, but let's do some benchmarks once it works.