Use fungible traits for treasury pallet

[tonyalaribe]

Background

The current currency traits are deprecated and there's currently a move to modify the existing pallets to support the fungible traits. paritytech/polkadot-sdk#226
This is an important step towards some more topics which are coming up, such as the Multi-Asset Treasury topic which will build on these fungible traits.

Review notes:

Since some of the fungible traits are fairly new and some methods appear to have similar uses, it's not always clear if i'm using the right traits and methods. For eg it's not always clear whether to use mint_into or set_balance in tests, as a replacement for Balances::make_free_balance_be()

It's also not always clear what the direct fungible equivalents of some Currency methods are. Eg Currency has deposit_into_existing and deposit_creating and only deposit exists under fungible with the Precision::Exact and Precision::BestEffort flags. Please keep this in mind while reviewing, to spot scenarios where this PR might be using the wrong fungible method equivalents.

The PR depends on the changes to the fungible traits in the gav-hold-freeze branch (#12951), and would have to be reanchored on master when that branch has been merged

[paritytech-cicd-pr]

The CI pipeline was cancelled due to failure one of the required jobs.
Job name: test-linux-stable-int
Logs: https://gitlab.parity.io/parity/mirrors/substrate/-/jobs/2487220

[vivekvpandya]

Thanks! @tonyalaribe this is very helpful PR to understand the whole change going on with #12951
Does this PR require any migration? like converting existing balances which has been set with lock()/reserve() to be compatible with freeze()/hold() ?

[tonyalaribe]

Thanks! @tonyalaribe this is very helpful PR to understand the whole change going on with #12951 Does this PR require any migration? like converting existing balances which has been set with lock()/reserve() to be compatible with freeze()/hold() ?

I believe so. But the need for migrations will be from the work on the balance pallet in #12951. So the migrations are more for the balance pallet than the treasury pallet.