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/generic-asset/src/lib.rs

@@ -1096,7 +1096,12 @@ where
 		Zero::zero()
 	}
 
-	fn transfer(transactor: &T::AccountId, dest: &T::AccountId, value: Self::Balance) -> Result {
+	fn transfer(
+		transactor: &T::AccountId,
+		dest: &T::AccountId,
+		value: Self::Balance,
+		_: ExistenceRequirement, // no existential deposit policy for generic asset
+	) -> Result {
 		<Module<T>>::make_transfer(&U::asset_id(), transactor, dest, value)
 	}