add .map_all_split/2 and unwrap utils

[sitch]

I've found cases where it's useful to combine OK.map_all/2 along with Enum.split_with/2 in order to explain failure cases for various evaluations. Things like mapping a cast/1 function where you might need to detail to the user what was wrong with some items.

I also added .unwrap/1, .unwrap_ok/1 and .unwrap_failure/1 as general conveniences.

[giddie]

I definitely think this library needs unwrap, but I think it would be better to make it less convenient to drop the success/failure information. I would prefer:

• unwrap/2: opposite of "wrap" - returns the value from a successful result tuple. If the result is a failure ({:error, ...}), then it returns the second parameter. I'm not in favour of nils generally, but probably most people would expect that the second parameter should default to nil.