Commit of partial work (this commit will be amended) to share with J...

...ames

        - during validation stage:
            Event 1: resource_valid
                - run on each Module in URL path
                - Does this event need a better name, like validate_resource ?

                - binders:
                    - cb_users_*
                        - load user_id into context
                    - cb_accounts
                        - load account_db/accout_id into context

            Event 2: validate
                - only run on last Module, like resource_exists

        If only resource_exists() method took Context as parameter, could fold o
`api_binary()` is an alias for `'undefined' | ne_binary()`, FYI.