Relaxin the PVP with regards to adding instances

[Evan Laforge]

> Assuming no one uses orphan instances, adding instances is always safe
> because you can only add instances for types from packages you depend
> on, which means that they can't be depending on your package in turn
> and have defined a non-orphan instance for a type or class defined in
> your package.

The thing is if a library is missing some standard instance like
Monoid it's very tempting to put in your own.  So the instances the
library author is most likely to eventually add in are also the ones
people are most likely to have worked around locally by adding their
own orphans.

E.g. Applicative for Parsec, Monoid for Text.PrettyPrint.Doc.