Cabal: versioning of the hooks API

[Simon Marlow]

Folks, 

I've made some changes that affect the hooks API.  It occurs to me that
changes in this API will automatically break any Cabal Setup.lhs that is
using the old API, and there isn't a good workaround, since when you say
'runhaskell Setup.lhs' you're at the mercy of whatever packages are
visible to the default compiler.

I wonder, should we explicitly version this API?  That is, should we
provide defaultMainWithHooks2, defaultMainWithHooks3...?  That would at
allow us some backwards compatibility, and make things break earlier and
more predictably if the required version of Cabal isn't available.  On
the down side, it requires us to support multiple versions of the API
simultaneously.  We could use DEPRECATED pragmas to encourage people to
update their Setup.lhs scripts.  Thouhts?

Cheers,
	Simon