ci: add github workflows

This PR adds Github CI workflows to replace appveyor.

@jonhoo's ci conf repo has a reasonable stub of CI workflows and configuration for rust crates. It includes config for dependabot, codecov.io, and workflows for checking format and clippy, testing on multiple OSes, determining the minimum supported rust version, checking for problems with minimal versions, and runs scheduled tasks to ensure that new rust versions and dependencies do not break this crate.

This is a merge using the --allow-unrelated-histories flag, that brings in the entire history of that repo. This enables the CI to be later merged if configuration changes are made upstream.

I've got a PR out against the upstream repo that adds a bit more documentation about each workflow https://github.com/jonhoo/rust-ci-conf/pull/10

Review Approach

When reviewing this change, there's two parts to consider - the history pulled in from the upstream repo which should probably not be squashed (and which contains the bulk of the implementation.

https://github.com/jonhoo/rust-ci-conf/commits/main

And then the changes specifically for strum which probably can be squashed into a single commit (though I have not yet done so as the commit history is relevant to understanding why specific choices were made in configuring CI

https://github.com/joshka/strum/compare/d668c3d314ede430deaab41e9248afbff73172da...joshka:strum:ci

Concerns:

1.32.0 doesn't seem to build on MacOS right now.
There is a fix for building no-std correctly (building strum_nostd_tests and strum together adds the std feature, for some strange reason without the change)
The appveyor config is a lot more granular around the build targets - I wasn't sure if this was necessary or whether the OS toolchain based approach was sufficient. Happy to fix this if needed.

Testing

I ran this pull request in my own fork at https://github.com/joshka/strum/pull/1/commits with success (except for the 1.32.0 issue on Macs)

Peternator7 / strum