Consider using extend for extension traits
We have a few extension traits in arti. git-grep 'impl.*for [A-Z][^a-z]'
finds some, and !514 (merged) introduces another.
IMO we should probably be using extend
for this (that's the most popular of the "make an extension trait" macros). Either for all our extension traits, or for the ones that can't be done simply with pub trait SinkExt: Sink { fn new_method(&self) { ...
as per !514 (comment 2805323)