overhauling chat for v3

[quartzjer]

Significant overhaul of the v2 chat channel to v3, it still serves the core usage patterns of rival messenger but is minimized in scope for private ephemeral 1:1/group conversation patterns.

• Base32 encoding of IDs (instead of hex)
• SipHash 64bit sequencing (instead of murmur3 32bit)
• message types are simplified to join, chat, and ack
• roster/participant state is binary
• all joins and roster changes are forward immutable (cannot remove/update once joined)
• message ID sequences are implicit, explicit did not reduce the required validation
• activity states included in a message and separated from receipt states (acks)

[quartzjer]

There's still implementation progress happening, but I believe the core spec changes have stabilized so this can be merged and improved as needed yet before v3-stable.