Import message structures.

Added the String abstraction to freeflow.

Imported port structures and MAC address structures from
the onf submission.

Renamed the freeflow_sys library to freeflow.
Importing actions.

Only imported the action stubs, not the complete
library yet.
Working on the message layer.

Pulling in more structures from the submission. Slowly
considering rewrites to make the implementation simpler, more
clear, and faster-compiling. A work in progress.
Small lexical changes.

Use ofp::to_view/from_view to avoid needing to prefix these
operations with ofp:: everywhere.