With steveicarus/iverilog#483 resolved, this workaround is no longer
necessary.

- add release.sh script to automate most of the process
- minor revisions to the unreleased changelog
- migrate away from deprecated release asset action

- build in CI using snapshots from lts-13 through lts-22 and nightly
- allow building with alex >= 3.2 and happy >= 1.19
- fallback to cabal version string with incompatible versions of githash

- refresh GitHub Actions versions
- manually install shUnit2 v2.1.8
- more portable usage check
- run-all.sh exits on ctrl-c

The latest verion of iverilog enforces declaration ordering more
strictly. Update a few test cases to match. sv2v still supports
out-of-order items on a best-effort basis.

I'm opting for iverilog's interpretation of the specifications here. The
commercial simulators I tested seem to agree.