5.3.0

This is primarily a maintenance release. There are many bug fixes, improvements to system call coverage, and incremental performance improvements. Thanks to all our contributors.

The rr sources, rr buildid and rr traceinfo commands have been added to make it easier for external tools to work with rr traces. rr ls has been added to improve management of traces.

rr no longer depends on Python 2. Python 3 is required.