* FF-2: refactor ff_withdrawal
- introduce ff_transfer_machine and new ff_transfer as an underlying process
for any withdrawal, deposit and transfer between wallets.
- rename apps/ff_withdraw -> apps/ff_transfer and make corresponding updates
including sys.config.
- rename fistful/src/ff_transfer.erl -> fistful/src/ff_postings_transfer.erl
* Introduce ff_instrument as an underlying abstruction for ff_destination and ff_source.
* Add ff_deposit
* Add ff_source
* Add FistfulAdmin thrift service
* Add transfer migration code
* Drop (de)hydration for now, we'll think about it later.
* Reduce boilerplate w/ the help of `ff_machine` though much to be done still.
* Drop half-baked `ff_machine` from ff_core
* Supply missing specs + fix marshalling types
* Update test fixtures
