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 <25483142+joepetrowski@users.noreply.github.com>

* Improve documentation and add test

* Update srml/balances/src/lib.rs

Co-Authored-By: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* Switch to changing Currency trait instead

substrate/srml/support/src/traits.rs

@@ -384,6 +384,7 @@ pub trait Currency<AccountId> {
 		source: &AccountId,
 		dest: &AccountId,
 		value: Self::Balance,
+		existence_requirement: ExistenceRequirement,
 	) -> result::Result<(), &'static str>;
 
 	/// Deducts up to `value` from the combined balance of `who`, preferring to deduct from the