Add additional CI via GitHub Actions.

[waywardmonkeys]

This adds CI via GitHub Actions for various targets, some of which haven't been tested via CI before.

• Autotools (Mac and Linux).
• cmake on iOS using Xcode.
• cmake on macOS using ninja.
• cmake on Linux using ninja, with both clang and gcc checked.
• cmake on Windows using MSVC (x64 and x86, Clang, ARM64, Windows Store).
• Documentation via Sphinx.
• Meson on macOS and Linux.
• MSBuild on Windows (x64 and x86).
• Swift on macOS and Linux.

There are some things that could be improved in subsequent PRs:

• Remove Appveyor config and README link.
• Remove TravisCI config. I left this as it contains the old config for coverage.
• Add code coverage again.
• Fix building ARM and ARM64 targets via MSBuild.
• Fix building ARM via cmake on Windows.
• Have Sphinx fail on warnings (but keep going), once Sphinx warnings are fixed.
• Add tests for Swift and run them.
• Add emscripten builds + tests, absorbing the other 2 workflow configurations.
• Make sure SIMD is built and tested where available.
• Add msys2 builds.
• Add Linux ARM64 builds.

[waywardmonkeys]

You can see all of the CI checks here in this PR as they're prefixed with "CI /"

[recp]

@waywardmonkeys many thanks for CI support 🤗 the PR is merged 🚀