This was prompted by discussion in #92, during which it emerged that
the docs were very unclear, and made AccumT appear much more generally
useful than it really is.

The `Foldable1` class (originally from the `semigroupoids` library) was added
to `base` in version 4.18.0.0 (GHC 9.6.1). The `semigroupoids` library defined
`Foldable1` instances for data types from `transformers`, so this patch moves
the corresponding instances to `transformers` itself to accommodate the
migration.

For more information on `Foldable1`, see this Core Libraries Committee
proposal:
https://github.com/haskell/core-libraries-committee/issues/9

See https://github.com/haskell-infra/hackage-trustees/issues/356

See https://github.com/agda/agda/issues/5659
Oleg has pointed out that GHC 8.6 has MonadFailDesugaring on by default,
which provides an alternative solution to the issue with MonadTrans
instances that led to this change.

Although this is available in GHC 7.2, GHC.Generics is not declared
Trustworthy in that version.

Before GHC 8.8 (base-4.13), the Monad class also included fail,
which might require more than Monad to implement.

conditional on GHC >= 8.6
(from SamuelSchlesinger and Icelandjack)

Suggested by Roman Cheplyaka.