Send is not implemented for RingBuf<T> where T: Send.

[dradtke]

My guess is that this is just an oversight from some of the recent changes. I found these two lines in the source file for Vec, which seem to fit with the recent change to make Send opt-in:

unsafe impl<T: Send> Send for Vec<T> { }
unsafe impl<T: Sync> Sync for Vec<T> { }

But no such impl statement exists in the source file for RingBuf. Considering how similar the struct definitions are for these collections, I assume there's no reason it couldn't be implemented for RingBuf as well.

[alexcrichton]

Dupe of #20322, but thanks!