Proposal: add -XPolyKinds to Data.Monoid

[Carl Howells]

At the moment, the Monoid instance for Data.Proxy.Proxy is only
defined for types with kind *.

    Prelude Data.Proxy Data.Monoid> (Proxy :: Proxy String) <> Proxy
    Proxy
    Prelude Data.Proxy Data.Monoid> :set -XDataKinds
    Prelude Data.Proxy Data.Monoid> (Proxy :: Proxy 5) <> Proxy

    <interactive>:8:20:
        No instance for (Monoid (Proxy 5)) arising from a use of ‘<>’
        In the expression: (Proxy :: Proxy 5) <> Proxy
        In an equation for ‘it’: it = (Proxy :: Proxy 5) <> Proxy

Enabling -XPolyKinds while compiling the Data.Monoid module is the
only change necessary to make the Monoid instance for Proxy polykinded
like Proxy itself is. I don't believe adding a language pragma would
have any other effect on that module, and I believe the only effect of
changing that instance would be allowing more correct programs to
compile.

This is my first proposal to this list, so I'm starting with something
that I hope isn't controversial. Let me know if I should do anything
different procedurally in future proposals.

Thanks.