Revert "Revert adding Atomic::from_mut."
Revert "Revert adding Atomic::from_mut."

This reverts commit 5ef1db3622c373883571868cbdafbfbd568cddcb.

aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust