Experimental message compression.
Add heading compression.
Hexid and squawk compression changes

* Encode all six characters of the hex ID.

* Encode the squawk code as two bytes of 4-character hexadecimal
Binary encoding reduces upstream bandwidth by 2/3rds

* common fields are binary-encoded and use shortened field names to achieve a 2/3rds reduction in upstream bandwidth

* version bump to 1.19
Send a compression version field with login info.
Merge 1.19 changes from upstream.
Pull in the 1.19-1 changelog.