Ryan Scott authored Jul 23, 2019 and Marge Bot committed Jul 25, 2019

Before, `type DefUses = [DefUse]`. But lists are a terrible choice of
data structure here, as we frequently append to the right of a
`DefUses`, which yields some displeasing asymptotics. Let's instead
use `OrdList`, which has constant-time appending to the right.

This is one step on the way to #10347.