Add Typeable constraint to a few LayoutClass instances

[liskin]

Description

This enables adding the Typeable constraint to LayoutClass itself
(xmonad/xmonad#242)
which in turn enables querying the current layout state. That might be
useful to e.g. show the current X.L.WorkspaceDir in xmobar.

Example usage: liskin/dotfiles@6ea6c52

Checklist

• I've read CONTRIBUTING.md

• I tested my changes with xmonad-testing

• I updated the CHANGES.md file

• I updated the XMonad.Doc.Extending file (if appropriate)

before merging, I need to finish the above checklist tasks, but I'd like to gather feedback first

[byorgey]

Seems reasonable to me.

[liskin]

I thought some more about this and decided that it's best if we merge this as is. This PR doesn't really change or enable anything, so it doesn't make sense to add documentation and changelog entry for it.

I changed the commit message to say:

This is a preparation commit that fixes the compile failures that would result in merging that change to xmonad. For this to be generally useful we first need to merge (and ideally also release) that xmonad change,
and then we'll need some documentation and perhaps a type class to help find the right LayoutModifier in the tree of ModifiedLayouts and Choices. That will come later.

So that's my plan now. Get these two preparatory changes merged and released in the right order, and then add a InspectLayoutModifier class, something along these lines:

class InspectLayoutModifier l a where
    inspectModifier :: (LayoutModifier m a, Typeable m) => l a -> Maybe (m a)

instance (LayoutClass l a, LayoutModifier m a, Typeable m, Typeable a) => InspectLayoutModifier (ModifiedLayout m l) a where
    inspectModifier l = case cast l of
        Just (ModifiedLayout m (_ :: l a)) -> Just m
        Nothing -> Nothing

If you still find my plan reasonable, I think this is now ready to be merged and xmonad/xmonad#242 can then be merged as well.

[byorgey]

I think it's still worth a brief note in the CHANGELOG, since in theory anyone using these instances may have things break if they don't already have a Typeable constraint.

[byorgey]

However, overall I think the plan sounds reasonable.

[liskin]

You've got a point. :-)

Here:

   * Several `LayoutClass` instances now have an additional `Typeable`
    constraint which may break some advanced configs. The upside is that we
    can now add `Typeable` to `LayoutClass` in `XMonad.Core` and make it
    possible to introspect the current layout and its modifiers.

force-pushed.