Add `ExistenceRequirement` to `Currency` trait (#4000)

* Added a public transfer_some function and a private transfer_inner fn

* Move transfer_some to the end of the module impl to fix failing contracts test

* Change whitespace

* Remove needless change to transfer logic

* Fix error

* Update srml/balances/src/lib.rs

Co-Authored-By: joe petrowski <[email protected]>

* Improve documentation and add test

* Update srml/balances/src/lib.rs

Co-Authored-By: Kian Paimani <[email protected]>

* Switch to changing Currency trait instead