Display build status.
Lift list of supported instances to top-level.
Improve error message about unknown PTS instance.
Handle errors in command-line parsing via ErrorsT.
Improve formatting of errors without position.
Activate fomega.
Add PTS instance synonym 'fomega'.
Fix orphan instances of MonadErrors

This also moves all its instances for monad transformers together, for
extra clarity. ErrorsT instances are separate.
Reduce cosmetic differences between instances

recover for StateT is actually different, so it's good if it looks
different.
Drop unused binding
Merge pull request #70 from Blaisorblade/topic/cleanup

Fix orphan instances of MonadErrors & misc
Use Bindings type synonym where possible.
Avoid writer monad (fix #72).

Commit 1c19757fdc6d156d326ae9b2dfb1c7a84c4f37a1 introduced a memory leak
by using a Writer monad to collect the exported bindings of a
module. This would create a big thunk (growing with the number of >>=
calls? arranging for lots of mappend mempty mempty to be executed in the
future?)

This commit attaches a boolean exported-or-not flag to every binding in
scope and thereby arranges for the exported bindings to be collected
with the state monad that is already in the monad transformer stack.

Thanks to Paolo for help with diagnosing the memory leak.