Haskell Platform call for consensus: add transformers and revise the mtl package to depend on it

[Ross Paterson]

Currently the following 32 library packages directly depending on mtl
are broken by the changes (including reasons and number of packages
that directly or indirectly depend on these):

applicative-extras-0.1.6              Applicative instance (9 clients)
blogination-0.5                       Applicative instance (no clients)
category-extras-0.53.5                Functor constraint   (15 clients)
csound-expression-0.0                 base monad members   (no clients)
derive-2.3.0.2                        Applicative instance (13 clients)
encoding-0.6.3                        base monad members   (15 clients)
FileManip-0.3.3.1                     base monad members   (13 clients)
FileManipCompat-0.15                  base monad members   (7 clients)
HaLeX-1.1.1                           base monad members   (no clients)
happstack-server-0.5.0.2              Functor constraint   (32 clients)
hashed-storage-0.5.3                  Functor constraint   (6 clients)
HaskellNet-0.2.3                      base monad members   (no clients)
hmk-0.9.7                             Applicative instance (1 clients)
hssqlppp-0.2.0                        Error instance       (1 clients)
jmacro-0.3.2                          base monad instance  (no clients)
kibro-0.4.3                           Functor constraint   (11 clients)
llvm-ht-0.7.0.0                       base monad members   (2 clients)
monad-param-0.0.2                     Functor constraint   (no clients)
MonadRandom-0.1.5                     Functor constraint   (13 clients)
nntp-0.0.3                            Functor constraint   (no clients)
OpenCLRaw-1.0.1001                    base monad members   (no clients)
open-witness-0.1.1                    base monad members   (no clients)
plot-0.1.2.3                          base monad members   (1 clients)
random-fu-0.1.0.0                     base monad instance  (3 clients)
redHandlers-0.1                       Functor constraint   (1 clients)
she-0.1                               Applicative instance (no clients)
swish-0.2.1                           base monad members   (no clients)
tls-0.2                               Functor constraint   (6 clients)
Twofish-0.2                           base monad instance  (no clients)
xmonad-contrib-bluetilebranch-0.9.1.4 base monad members   (1 clients)
yhccore-0.9.1                         base monad instance  (3 clients)

A further concern is that we now have 138 packages whose dependencies
exclude mtl-2 and 8 (and rising) whose dependencies require it.
That will prevent an increasing number of packages from building.