Split into LevelUp proxy wrapper, and Merkle LevelDown wrapper

[mappum]

This module really does two things:

• Handles mutations on a proxied object and commits mutations to a LevelUp db
• Implements a LevelUp interface which wraps a LevelUp db and organizes data into a Merkle tree

We could split these into 2 packages, levelize and merklevel since they can each be useful without the other.

[mappum]

Also makes sense to make a third module to handle mutation transactions/commits, which can unbundle the commit/rollback logic from the levelize data structures, and will be useful in state management inside lotion.

[mappum]

The commit/rollback logic is now extracted and improved in a new package called muta.

merk doesn't currently make use of this, but that will happen in the future.

[mappum]

Closing since this repo was already refactored to only include the merkle kv store component.