Stabilize SBP protocol

[mfine]

https://swiftnav.hackpad.com/SBP-Messages-Status-XS5xpWwhzwd
https://swiftnav.hackpad.com/SBP-Messages-erring-with-MSG_PRINT-jX8Do75gKC0

The SBP messages have been reviewed and catalogued, along with those messages using print to signal errors. Work towards promoting more of them.

[mfine]

Put together a versioning proposal for SBP:

https://swiftnav.hackpad.com/SBP-Message-Versioning-z1J4eNjbVok

[mfine]

Added issues to promote classes of messages to stable along the guidelines of our new versioning policy:

#165
#166
#167

[mfine]

Completion criteria: stabilize settings, logging, and file_io packages, and lay out versioning containers plus policy.