why: Cut alpha.16 from this pull request: the first release to
publish tmux-workspace, as libtmux-workspace-cli, and the first built
on master's workspace work.
what:
- Date the alpha.16 changelog section and move the install snippets
- Advance the development version to alpha.16-SNAPSHOT
- Set the API baseline to the published alpha.15 release, recording its
artifacts' checksums and dropping alpha.14's
- Split MIGRATION.md by release, which it had never been, and say in
RELEASING.md that a release moves its entries under its version