Add a #:cabal-revision argument for specifying which Cabal file revision
from Hackage should be used.
* guix/build-system/haskell.scm (source-url->revision-url): New function.
(lower): Accept a cabal-revision keyword argument, convert it to an
origin record, and add it to the resulting bag's host-inputs.
(haskell-build): Pass the cabal-revision input to the builder as an
argument.
* guix/build/haskell-build-system.scm (patch-cabal-file): New phase.
(%standard-phases): Add it.
