Re-exporting modules in the Foreign hierarchy

[Sven Panne]

Yet another change request for the FFI libraries: For reasons of
consistency, I propose to introduce an intermediate module
Foreign.Marshal, which re-exports the modules:

    Foreign.Marshal.Alloc
    Foreign.Marshal.Array
    Foreign.Marshal.Error
    Foreign.Marshal.Pool
    Foreign.Marshal.Utils

While I'm at it: What was the reason for the decision that Foreign.C
re-exports

    Foreign.C.Types
    Foreign.C.String
    Foreign.C.Error

but not Foreign.C.TypesISO? If there wasn't a good one, I propose to
add this for consistency again. It would be nice if modules exported
at least all the ones directly below them in the hierarchy.

Cheers,
    S.