Build binaries in CI and upload as release assets

[rafaelmaeuer]

As suggested in magiclen#12 (comment), prebuild-binaries should be generated in Github Actions CI (e.g. https://github.com/actions-rs/cargo#cross-compilation) and uploaded as release assets afterwards: Can you add binaries to releases with GitHub Actions?

But this requires changes in installation process (download prebuild-binaries from github on install) and ensure, cross-compilation works in CI for all currently supported targets and platforms (actions-rs/toolchain#216 seems to be unmaintained, using dtolnay/rust-toolchain now).