Add Monad, export Monad, Functor and Either

[dpvanbalen]

Description
Adds a Monad typeclass, and exports two modules (Functor and Either) which I was surprised to see unexported.

Motivation and context
While toying with a little Accelerate project, playing around with the pattern synonyms, I found myself implementing (>>=) for Maybe, and was surprised to see it not in Accelerate.

How has this been tested?
The Show instance looked reasonable.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist
Go over all the following points, and put an x in all the boxes that apply. If you're unsure about any of these, don't hesitate to ask. We're here to help!

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

[tomsmeding]

Just passing by, looks interesting!

• The @since 1.2.0.0 in Monad.hs should probably be 1.3.0.1, or whatever is to be the next version. Don't know that the protocol is here.
• Is RebindableSyntax really necessary in Monad.hs? Looks like a perfectly normal file.
• No Applicative?

[dpvanbalen]

Just passing by, looks interesting!

* The `@since 1.2.0.0` in Monad.hs should probably be 1.3.0.1, or whatever is to be the next version. Don't know that the protocol is here.

Oops, I copied that part from Functor. I guess 1.4.0.0 is the next version?

* Is `RebindableSyntax` really necessary in Monad.hs? Looks like a perfectly normal file.

Another oops, that is not needed.

* No `Applicative`?

Sadly no, we can't follow the MAP because there's no Elt (a -> b) instance. (Of course, that doesn't stop you from writing an Applicative class, you just can't use it. :) )

[tomsmeding]

Of course, that doesn't stop you from writing an Applicative class, you just can't use it. :)

There are limits to the amount of useless work we can get away with as academics. Makes sense :)

[dpvanbalen]

Should the documentation reflect the lack of Applicatives? I suppose it's not obvious why it's missing, and it might cause confusion.
On another note, we could ofcourse keep adding these 'extended prelude'-type functions, datatypes, and classes, but maybe a whole 'MTL port' or 'Data.Bifunctor' should be in its own package or a separate module.

[tmcdonell]

brilliant, thanks! (: