Initial (optional) `triomphe::Arc` support

[virtualritz]

See subject.

This adds support for swapping out std::sync::Arc (parking_lot) resp. alloc::sync::Arc (lock_api) for triomphe::Arc.

Both crates had a triomphe feature added but alternatively I could put this behind a triomphe_arc_lock feature instead. Or even arc_de_triomphe_lock, to fully express the presumed naming intent of that crate. :wink:

I need this for a project I'm working on so I thought I may as well open a PR, in case this is useful. If not, just close the PR and ignore.