Separate warnings for unused local and top-level bindings

I'd like separate warnings for local and top-level
unused binds.  I often have "unused" top-level
functions which I use from ghci.  I'd like to avoid
warnings for
these while retaining the warnings for unused local
bindings (which can always be eliminated by prefix the
name with an underscore.)

Edited Mar 09, 2019 by byorgey

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