Automatic merge from submit-queue
Round should avoid clearing s, save a string
Instead of saving bytes, save a string, which makes String() faster
and does not unduly penalize marshal. During parse, save the string
if it is in canonical form.
@wojtek-t @lavalamp this makes quantity.String() faster for a few cases
where it matters. We were also not clearing s properly before on Round()
3717e4e46b