The `MonadException` class was removed in favor of `MonadCatch`, so we
can remove its instances, enabling us to remove the library dependency
on haskeline.
To simplify dependency resolution with GHC < 8.10, this also disables
the executable in that configuration.
Note that this is an enforced public API change.
I went with changing the `MonadFile` typeclass rather than threading
through a `MonadFail` at every use-site, but that would be possible as
well if it makes more semantic sense - however, since `MonadFile`
already seems to require an IO-ish implementation, adding that
superclass doesn't seem like an imposition.
