Implement -XMonoLocalBinds: a radical new flag

Implement -XMonoLocalBinds: a radical new flag

The new flag -XMonoLocalBinds tells GHC not to generalise nested
bindings in let or where clauses, unless there is a type signature,
in which case we use it.

I'm thinking about whether this might actually be a good direction for
Haskell go to in, although it seems pretty radical.  Anyway, the flag
is easy to implement (look at how few lines change), and having it
will allow us to experiement with and without.

Just for the record, below are the changes required in the boot
libraries -- ie the places where.  Not quite as minimal as I'd hoped,
but the changes fall into a few standard patterns, and most represent
(in my opinion) sytlistic improvements.  I will not push these patches,
however.

== running darcs what -s --repodir libraries/base
M ./Control/Arrow.hs -2 +4
M ./Data/Data.hs -7 +22
M ./System/IO/Error.hs +1
M ./Text/ParserCombinators/ReadP.hs +1
== running darcs what -s --repodir libraries/bytestring
M ./Data/ByteString/Char8.hs -1 +2
M ./Data/ByteString/Unsafe.hs +1
== running darcs what -s --repodir libraries/Cabal
M ./Distribution/PackageDescription.hs -2 +6
M ./Distribution/PackageDescription/Check.hs +3
M ./Distribution/PackageDescription/Configuration.hs -1 +3
M ./Distribution/ParseUtils.hs -2 +4
M ./Distribution/Simple/Command.hs -1 +4
M ./Distribution/Simple/Setup.hs -12 +24
M ./Distribution/Simple/UserHooks.hs -1 +5
== running darcs what -s --repodir libraries/containers
M ./Data/IntMap.hs -2 +2
== running darcs what -s --repodir libraries/dph
M ./dph-base/Data/Array/Parallel/Arr/BBArr.hs -1 +3
M ./dph-base/Data/Array/Parallel/Arr/BUArr.hs -2 +4
M ./dph-prim-par/Data/Array/Parallel/Unlifted/Distributed/Arrays.hs -6 +10
M ./dph-prim-par/Data/Array/Parallel/Unlifted/Distributed/Combinators.hs -3 +6
M ./dph-prim-seq/Data/Array/Parallel/Unlifted/Sequential/Flat/Permute.hs -2 +4
== running darcs what -s --repodir libraries/syb
M ./Data/Generics/Twins.hs -5 +18