Upgrade MaybeT to AMP, and otherwise clean it up

[Ross Paterson]

On Sun, Jan 04, 2015 at 02:18:55AM +0000, Ross Paterson wrote:
> On Sat, Jan 03, 2015 at 03:31:29PM -0500, David Feuer wrote:
> > http://stackoverflow.com/questions/22750315/
> [...]
> (The instance proposed at the above link doesn't meet this expectation;
> it is equivalent to Errors ().)

Correction: it is equivalent to Compose Maybe