Trac #13064: push

Ben I've pushed fixes for Trac #13064/#15393, and also for #15487. The fixes are on wip/T13064 They concern error reporting for unused imports etc. I have not pushed to master because the fixes produce warnings in at least * containers * bytestring * text about unused imports. And indeed they are unused! But I'm not sure I'll the protocol right about updating those package, doing the right submodule updates etc. Also I have not compiled Haddock. Could you possibly close the loop on this - do the necessary changes and commit to master? Thanks! Simon