Named reserve (#7778)

* add NamedReservableCurrency

* move currency related trait and types into a new file

* implement NamedReservableCurrency

* remove empty reserves

* Update frame/support/src/traits.rs

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

* fix build

* bump year

* add MaxReserves

* repatriate_reserved_named should put reserved fund into named reserved

* add tests

* add some docs

* fix warning

* Update lib.rs

* fix test

* fix test

* fix

* fix

* triggier CI

* Move NamedReservableCurrency.

* Use strongly bounded vec for reserves.

* Fix test.

* remove duplicated file

* trigger CI

* Make `ReserveIdentifier` assosicated type

* add helpers

* make ReserveIdentifier assosicated type

* fix

* update

* trigger CI

* Apply suggestions from code review

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

* trigger CI

* Apply suggestions from code review

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>
Co-authored-by: Gavin Wood <i@gavwood.com>
Co-authored-by: Shaun Wang <spxwang@gmail.com>