Schema

This makes the schema handling a little cleaner in simplebridge by introducing a scoped type similar to libpack's scoped DB type. This just manages/enforces the schema within the simplebridge driver and is not intended to be used externally.

Reviews (and opinions on this) are very welcome. #129 should be reviewed before this PR as it contains the commits from #129 

@shykes @icecrime @lk4d4 @tiborvass