Plug testing gaps

[treeowl]

#25 adds MonadReader and MonadError instances. It includes a couple unit tests for MonadReader borrowed from a logict bug report, but no general property tests. It has no tests whatsoever for MonadError. What I think we want:

1. Representations of the free MonadReader and the free MonadError we can generate for testing, if indeed such structures exist.
2. Tests hopefully verifying that the instances obey reasonable laws as discussed in Missing laws haskell/mtl#5.
3. Tests hopefully verifying that the instances behave the same as the ones in logict, and if not a determination of which ones are correct.

Additionally: we should test the composition law for hoist.

[dagit]

We also want a test simple for this: atzeus/reflectionwithoutremorse#6

[treeowl]

#25 now includes tests for the MonadPlus laws, so I think that'll cover the bug you found in reflectionwithoutremorse.

[treeowl]

We need a test showing that read . show = id. I made an attempt, which failed mysteriously. I'll keep working on it.