Ben Gamari authored Nov 10, 2019 and Marge Bot committed Dec 03, 2019

This does two things:

 * Eliminate all uses of Unique.deriveUnique, which was quite easy to
   mis-use and extremely subtle.

 * Rename the previous "derived unique" notion to "local unique". This
   is possible because the only places where `uniqAway` can be safely
   used are those where local uniqueness (with respect to some
   InScopeSet) is sufficient.

 * Rework the implementation of VarEnv.uniqAway, as discussed in #17462.
   This should make the operation significantly more efficient than its
   previous iterative implementation..

Metric Decrease:
    T9872c
    T12227
    T9233
    T14683
    T5030
    T12545
    hie002

Metric Increase:
    T9961