Miller 6 release candidate 1

This is an update after https://github.com/johnkerl/miller/releases/tag/v6.0.0-beta, including several performance-optimization PRs since then.

This is a release-candidate tag -- it doesn't include #827 or #755 both of which are blockers for the Miller 6.0.0 release per se.

The main purpose is for a Conda build by @BEFH as tracked at #372 (comment).

After #755 and #827 are resolved we will have either 6.0.0.rc2 (if other issues arise) or simply 6.0.0.