Replace file watcher

[casselc]

This attempts to replace beholder with a java.nio.file.WatchService implementation to address the issues described in #7. I've kicked the tires on Windows and Linux aarch64, unable to test on Mac.

[plexus]

The main reason we use beholder, and the reason beholder exists, is for better mac support, so unless some mac people provide experience reports (regular and M1) I won't be able to merge this.

[plexus]

There are also several unrelated changes in here, including making public vars private, and adding hashing and denouncing. These should be in separate PRs. Thank you!