Added "do notation" for Maybe, Either, Validation and IO Monads using generators

[sevcsik]

Added a generic doNotation function, which is bound to monads Maybe, Either, Validation and IO as a static do method. This will allow one to have a similar experience as Haskell's do using ES6 generators.

Example:

Maybe.do(function*() {
  var a = yield Maybe.Just(5)
  var b = yield Maybe.Just(1)
  return a+b
}) // => Just(6)

The new library code itself doesn't require ES6, but the unit tests do, thus it was required to drop PhantomJS in favour of headless Chrome for unit tests (PhantomJS is abandoned anyway).

This PR doesn't include TypeScripy bindings yet, as it would require the IterableIterator type which would make the es6 target mandatory.

Fixes #103

[graingert]

You could use Babel to support phantomjs

[sevcsik]

@graingert, I could, but I find it overkill to introduce a transpiler just to be able to run the unit tests. As PhantomJS has been abandoned, I think it makes more sense to switch to a headless browser.

[sevcsik]

@ulfryk?

[ulfryk]

Sorry guys, don't have too much time lately…
👍 for HEadlessCHrome
👎 for Babel

And what is most important from my point of view, this should be a separate library in @monet/ NPM namespace. And it's source should be written in TS (as we are planning to rewrite monet itself), and lib/ should be transpiled to ES6 in this case. Tests (both for node and browser) should be also written in TS for sake of consistency.

I;ll bootstrap repository for it today/tomorrow ;)

[ulfryk]

@sevcsik - sorry for horrible delay :(

Are you still willing to contribute and make a standalone lib for do ?

[sevcsik]

@ulfryk I did a PoC for Maybe last year. TBH I don't remember how far I got. I didn't end up using Monet in any of my projects so I put this on the backburner.

Here's what I got: https://github.com/sevcsik/monet-do/blob/master/src/index.ts

If you see potential in it I'd be more than happy to continue. I'll write some tests to see if it works like the JS version.