Missing Default for std::sync::Condvar

[vi]

It has fn new(), so there should be no problem for Default.

See also: https://github.com/Manishearth/rust-clippy/issues/705

[llogiq]

I'm going to hijack this issue and add missing Default impls for Mutex<T> and RwLock<T> where T: Default.

I'll have a PR ready shortly.

[tbu-]

Fixed by #32785.