Makes several corrections to post 59, and adds example code with tests
to ensure the post content is correct. In particular:

* links to code early in the post
* adds necessary imports early in the post
* removes `Alternative` instance for `Xor`
* rename `f` and `f2` to more descriptive names
    * avoids name-shadowing later
* fixes the implementation of most Validation helper functions
    * previously returned: `[Validation NumberError a]`
    *              needed: `Validation [NumberError] a`
* corrected the subsequent examples
* fixed one erroneous example
* spelling: let's -> lets