Update SRML Assets: add total supply query, refactor with specific tests (#1185)

* Update SRML Assets: add total supply query, refactor with specific unit tests, update assertions

* Add feature and tests to allow querying total supply

* Add assertion and tests to ensure that transfer amount is greater than or equal to one unit

* Replace broad `it_works` function test with various specific unit tests

* Fix `destroy` function by moving assertion before the action

* Fix typos `Transfered` should be `Transferred`, `requried` should be `required`

* Reference: https://hackmd.io/nr6kPD2sR4urmljtvHs0CQ?view#Assets-Module

* refactor: Order imports alphabetically

* review-fix: Replace non-zero check with shorter equivalent

* review-fix: Restore order of non-zero assertion and destroy account

* Update lib.rs