* Error type for MultiCurrency trait and tokens module.

* fmt

* Update error type.

* Update transfer unit test.

* Tokens module unit tests.

* Remove workspace src directory

* Add example crate.

* fmt

* Update example.

* Minor fixes.

* Remove allow dead code.

* Define MultiCurrency trait.

* Declare storage.

* Impl multi currency trait for tokens module.

* Balance transfer.

* Fix typo.

* fmt

* Add transferred event.

* Update tokens trait and slash impl.

* Use value type for currency id params.

* fmt