Fix rust send sync

[aseyboldt]

Fixes #258 by explicitly marking the StanLibrary as Send + Sync.

This also adds an entry to the github workflow, which should automatically find most backward incompatible changes that break semver.

[aseyboldt]

I think I found the reason (or part of it), why rust thought this wasn't threadsafe.
The library contains version numbers as int, not const int, so two threads could modify the version number concurrently in an unsafe way.
This only showed up now, because the version numbers were not exposed to rust in the old brindgen version, but with 0.71 they are.
I changed the versions to const, and I think this should now automatically make StanLibrary Send + Sync, but there seems to be a bindgen issue (rust-lang/rust-bindgen#3059), and with this it is still exported as a mut int.
So I left the explicit Send + Sync implementation in. Since the wrapper never modifies the version (that would be fun) I think this is fine.

[WardBrian]

(Sorry, fat thumbs!)

Good catch, and I think this is the correct fix.

We can release a 2.6.1 to patch this. The merge of #257 makes this a bit annoying, since we probably don’t want to change the Julia LTS in a minor version. We can try to rig up something to skip releasing Julia for this patch or I can do some cherry picking on another branch after the holidays

[aseyboldt]

Sounds good, thanks.
And (slightly early) Merry Christmas :-)