Split Monad.fs for easier inclusion of specific computation expressions via paket

[panesofglass]

See https://github.com/fsprojects/fsharpx/tree/master/src/FSharpx.Core/ComputationExpressions

I started this long ago but never finished as we didn't have a way to select individual files. Paket now makes that possible!

[forki]

Yes most of the fsharpx stuff should be mini paket source repos

[panesofglass]

@forki, do you mean that we should create separate repos or just that separating the files would enable "mini paket source repos"?

[panesofglass]

Fixed in #364