Improve event json metadata (#710)

* Introduce `impl_event!` for declaring the Event/RawEvent for a module

* Implements a function for generating the event json metadata

* Update runtime `json_metadata` to include the new event json metadata

* Switch to `impl_event!` for new modules

* Remove unused module declaration in `impl_event!`

* Rename `impl_event!` to `decl_event!`

* Fixes some style nits