Make extendr not thread-safe

[CGMossa]

• Remove all usage of single_threaded
• Set --test-threads=1 always
• No longer rely on ownership tracking across threads, i.e. use thread_local! for Ownership.

[CGMossa]

My issue on Windows, is that the CI already inserts an [env] in the .cargo/config.toml.

[CGMossa]

Dear @yutannihilation. This isn't ready for final review yet, but I do want your guidance on this PR.

Basically, I got fed up chasing my own tail, and really embedding unnecessary bottlenecks to extendr, without
really paying it off properly. So I thought a branch where we see how it would look without upfront promise
of thread-safety.