release 0.8.4

[mr-c]

• [x] write release notes using the Release Notes Helper
• [x] bump version in meson.build to an even minor release number
• [x] bump SIMDE_VERSION_{MAJOR,MINOR,MICRO} in simde/simde-common.h to match
• [x] make release candidate (make sure to mark it as a "pre-release" in GitHub)
• [ ] test on Debian against all official and unofficial architectures using their default gcc version
• [ ] Rebuild all packages in Debian that use SIMDe in experimental to see if there are any regressions.
• [ ] fix or ignore any errors found
• [ ] make another release candidate (repeating the above as necessary)
• [ ] make final release
• [ ] upload an amalgamated build of the headers
• [ ] create a matching tag for simde-no-tests
• [ ] publish a blog post using the release notes
• [ ] email the mailing list about the blog post
• [ ] ping vcpkg to update, example
• [ ] post-release: bump version in meson.build to an odd minor release number
• [ ] post-release: bump SIMDE_VERSION_{MAJOR,MINOR,MICRO} in simde/simde-common.h to match
• [ ] copy this checklist to the wiki for next time

[mr-c]

Pre-release is out: https://github.com/simd-everywhere/simde/releases/tag/v0.8.4-rc1