Duplicated User type for inner events

[mzduke]

Within the file slackevents/inner_events.go defined a User type, which is used only for a couple of events.
There is already defined a User type (users.go`) which match data type from API reference: https://api.slack.com/types/user.
It makes implementation of event processing dreaded (force us to support 2 different data types with the same meaning).

Is there any reason to have two different data types?