Build failures with mtl-2.3

[Simon Jakobi]

src/Weigh.hs:161:3: error:
    Variable not in scope: unless :: Bool -> IO () -> IO a0
    |
161 |   unless
    |   ^^^^^^

As a Hackage trustee I have revised the affected versions on Hackage. See e.g. https://hackage.haskell.org/package/weigh-0.0.16/revisions/.