impl Default for CondVar, Mutex and RwLock

[llogiq]

this fixes issue #31865

Note: The impls are unstable for now.

[rust-highfive]

r? @brson

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[brson]

Looks related to #32785

[llogiq]

It is related. And #32785 goes even further than this.

[durka]

Note: The impls are unstable for now.

That doesn't work for trait impls, right?

[retep998]

@durka trait impls are insta-stable. Marking them unstable doesn't really work.

[llogiq]

OK then. I'll remove the unstable attrs if there is still interest.

[alexcrichton]

Thanks for the PR @llogiq! I believe these are all covered by #32785, so I'm going to close in favor of that.