threads: add feature flags

[abrown]

This adds both the Cargo-level and CLI-level flags for the shared-everything-threads proposal. We may not want to expose this feature just yet, but I need a place to park this since the stack of commits to enable shared-everything threads is getting out of hand. The feedback I'm look for is: what we should be gating with this new Cargo feature?

[github-actions]

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[alexcrichton]

What do you think about using the preexisting threads Cargo feature for this? That's mostly avoiding runtime deps of threads/libstd/etc and having that gate both threads and shared-everything-threads seems reasonable to me

[alexcrichton]

Oh and do directly answer this:

The feedback I'm look for is: what we should be gating with this new Cargo feature?

I wrote up some basic thoughts here but the tl;dr; IMO is "anything that requires a runtime dependency" aka something outside of libcore. In that sense the current threads proposal is gated because it requires mutexes/etc to implement atomic notify/wait. I don't think that shared-everything-threads will require any more than threads, though, but any nontrivial dependency above the core crate should be gated.

(or do you have specific things you're curious about too?)