Validation done in commanded middleware

[revati]

Main changes:

• commands are created with ExConstructor (similarly how its done with events new/1) without validation and any casting at that point
• commands have validate function which takes struct data and validates it
• commands do not import all Ecto.Changeset, but only needed functions
• validation done in middleware (seems to be more idiomatic way how to integrate with commanded)
• • on failure: halts command execution, returns changeset
• • on success: applies changes and updates command values, proceeds
• removed event version specific handling as it can be done manually for those who please.
• events have explicit test for default values
• contains: https://github.com/trbngr/commanded_messaging/pull/3