Redo how we manage deposits

[TimSheard]

This PR explores one mechanism for tracking deposits for each Stake credential and each Pool.
Motivated by #3113

Here is what happened
Added deposits (key deposits) to DState and (pool deposits) PState.
Functions that computed deposits, obligation, and refunds (keyRefunds
and totalDeposits, obigation etc) were changed to take DPState as input
Added DPState to UtxoEnv replacing pool stake. Incorporated TreeDiff
to show differences in two NewEpochState's. Gathered all the TreeDiff
stuff into one module Cardano.Ledger.TreeDiff in cardano-binary. This
includes all needed orphan instances from external types. Importing this module
should supply everything needed, and carry the orphan instances.
Added (.->>) as a version of (.->) that uses tree diff in small-steps-test
Added property tests that test the new invariants that must hold
1) utxosDepoits == sum (dsDeposited) + sum (psDeposited)
2) dom rewards == dom dsDeposited

[lehins]

I haven't looked through the full PR yet, here are my comments thus far

[teodanciu]

Went through all the code, added some small remarks and asked (likely stupid) questions mostly for my education .
Had no idea this change would impact so much of the codebase and how many things would have to be adapted to the change, a lot of work!

[lehins]

Here are some more comments and suggestions

I don't fully grasp the logic in this PR, since I had limited exposure to staking and delegations, however from what I do understand I don't see any real problems.

[aniketd]

👍

[lehins]

Awesome work!!!