Multi-package targets, more constructors found

What's Changed

• 'fzgen ./...' now works, placing output files in directory of target packages #11
• A wider set of candidate constructors are examined by default #16
  • Previously, the regexp '^New' was the default, but now '.' is the default -ctor regexp
  • Hopefully the -ctor flag is now less commonly used
• Multiple chains for a single package are now created #12
• Bug fixes that enable more targets to work by default #9
• Performance improvements
• Other smaller improvements, such as better messages explaining why a target package was skipped, and better emitted comments.

Thanks especially to @josharian (Tailscale) for the helpful bug reports!

Full v0.4.2 Changelog: v0.4.1...v0.4.2

Please see the v0.4.0 release notes for larger recent updates.