Add maybe utils

[TimWhiting]

A few updates to the standard library for maybe. Open to feedback / suggestions.

I've improved the error location for the unjust function. I also renamed unjust2 to unjust, since none of the other functions in core/maybe2 use the 2 qualifier, and we have static overloading which can resolve this most of the time.

I then also added an expect function which expects an explicit error message as a second parameter.
This mimics the naming of this function in Rust's Option type, but I'm open to other names.

If we go with Rust's naming for this it might be good to look at also changing unjust to unwrap - maybe with a deprecation notice on the old definition due to it's wide use. Haskell uses fromMaybe, Swift uses a postfix ! and unsafelyUnwrap for no runtime checks in non-debug builds.