This is an implementation of the deprecated exports proposal #134.
The proposal introduces an ability to introduce warnings to exports.
This allows for deprecating a name only when it is exported from a specific
module, rather than always depreacting its usage. In this example:

    module A ({-# DEPRECATED "do not use" #-} x) where
    x = undefined
    ---
    module B where
    import A(x)

`x` will emit a warning when it is explicitly imported.

Like the declaration warnings, export warnings are first accumulated within
the `Warnings` struct, then passed into the ModIface, from which they are
then looked up and warned about in the importing module in the `lookup_ie`
helpers of the `filterImports` function (for the explicitly imported names)
and in the `addUsedGRE(s)` functions where they warn about regular usages
of the imported name.

In terms of the AST information, the custom warning is stored in the
extension field of the variants of the `IE` type (see Trees that Grow for
more information).

The commit includes a bump to the haddock submodule added in MR #28

Signed-off-by: Bartłomiej Cieślar <bcieslar2001@gmail.com>