Move LockableCurrency trait to fungibles::Lockable and deprecate LockableCurrency (#12798)

* WIP move LockableCurrency to fungibles

* rename Lockable and LockIdentifier to funginbles::*

* fix imports further

* change Lockable from fungible to fungibles

* reintroduce LockableCurrency but marked as deprecated

* fix imports

* fix imports

* cargo fmt

* add allow deprecated warnings

* remove unused benchmark import

* fix some of the docs

* fix failing doctest check

* reexport LockIdentifier and LockableCurrency from support/traits

* reexport LockIdentifier and LockableCurrency from support/traits

* allow using deprecated re-export

* replace LockableCurrency and LockIdentifier with a module alias

* Update frame/support/src/traits/tokens/fungibles/lockable.rs

* Update frame/staking/src/pallet/mod.rs

Co-authored-by: Squirrel <gilescope@gmail.com>

* Update frame/support/src/traits.rs

Co-authored-by: Squirrel <gilescope@gmail.com>

* REVERT removing fungibles::Lockable import

Co-authored-by: parity-processbot <>
Co-authored-by: Squirrel <gilescope@gmail.com>