release 0.8.4

[mr-c]

• write release notes using the Release Notes Helper
• bump version in meson.build to an even minor release number
• bump SIMDE_VERSION_{MAJOR,MINOR,MICRO} in simde/simde-common.h to match
• 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