Relax instance requirements for bind/monad

[parsonsmatt]

For Cofree f to be a monad, you only need Alternative, not MonadPlus. From hackage:

Furthermore, if the functor f forms a monoid (for example, by being an instance of Alternative), the resulting Comonad is also a Monad. See Monadic Augment and Generalised Shortcut Fusion by Neil Ghani et al., Section 4.3 for more details.

The implementation of pure is an infinite loop. This PR also addresses that, and includes an Alternative requirement on the Applicative instance.

[paf31]

@garyb Can we merge this for 1.0 please? The Applicative instance which is there now actually looks wrong.

[garyb]

Sure thing, not sure how I missed this.

[parsonsmatt]

Thanks! 😄