5.0.0

Introduction of Cap'n Proto to stabilize the recording format. Recordings created in this rr release should be replayable in any future rr release.
New rr pack command makes recordings self-contained.
Recordings can be made on one machine and replayed on a different machine by trapping CPUID instructions when supported on the replay machine.
Brotli compression for smaller traces and lower recording overhead.
rr replay command line argument -- can be followed by any number of arguments to be passed to the debugger.
rr now chooses replay as the default subcommand if the first argument is a directory.
Performance improvements.
Syscall support expanded.
Many bugs fixed.