* Impl BasicCurrency & BasicCurrencyExtended.

* Add currencies module.

* Minor fixes

* Add equality bounds for CurrencyId.

* Currencies: add transfer dispatch.

* Currencies: impl BasicCurrencyExtended.

* Add unit tests for currencies module.

* More unit test for currencies module.

* Currencies module: add NativeCurrency type.

* Update unit tests for native currency.

* Add BasicCurrency impl for any type which implements srml_support::traits::Currency.

* Fix typo.

* Add BasicCurrencyAdapter.

* BasicCurrencyAdapter unit tests.

* Update BasicCurrencyAdapter.

* Update slash impl for BasicCurrencyAdapter.