Make fromLeft/fromRight total functions by providing default values

[JordanMartinez]

Fixes #47

[JordanMartinez]

Had to force push this because I didn't implement fromRight correctly due to putting the Right b pattern match after the left's _ pattern match

[hdgarrood]

I don't think it will be totally clear what the "default value" is when these documentation comments are rendered on Pursuit and the definition isn't visible. How about this instead?

A function that extracts the value from the Left data constructor. The first argument is a default value, which will be returned in the case where a Right is passed to fromLeft.

[JordanMartinez]

Yeah, that reads better. I've updated the docs to include that.

[JordanMartinez]

what if we leave old partial functions, but rename them to unsafeFromLeft and unsafeFromRight?

I think keeping them makes sense.

[garyb]

I dunno, you can just do fromLeft crash with this version 🙂