add deprecation warning for using `Poly`

[thautwarm]

I got ERROR: LoadError: UndefVarError: Poly not defined in my old code. If possible I hope I can get some notifications(at least warnings) instead of a sudden API change.

[thautwarm]

and also for polyint

[jverzani]

Thanks for writing. There was a very short (too short?) deprecation phase before v1.0. Now there is support for legacy code by calling using Polynomials.PolyCompat after loading Polynomials. It is documented, but a bit hard to find.