Fix BitSeq Snoc instance

[blaxill]

This is covered by tests but it appears they are not functioning correctly- only single item lists are generated in test/HaskellWorks/Data/RankSelect/BitSeqSpec.hs. E.g. try printing bs under snoc test:

  it "(|>) should snoc" $ requireTest $ do
    b         <- forAll $ G.bool
    bs        <- forAll $ G.list (R.linear 1 1000) G.bool
    ps        <- forAll $ pure $ BS.fromBools bs

    liftIO $ print bs -- Only ever a single element

Using resize results in lists greater than 1 element and causes the test to fail (without this PR)

  it "(|>) should snoc" $ requireTest $ do
    b         <- forAll $ G.bool
    bs        <- forAll $ G.resize 100 $ G.list (R.linear 1 1000) G.bool
    ps        <- forAll $ pure $ BS.fromBools bs

    liftIO $ print bs -- Now many elements

I'm not sure on the correct fix for the tests as I am less familiar with hedgehog, but there seems to be some non obvious interaction with hedgehog 'Size'.