hamming: Reimplement adhering to standard definition

As noted in https://github.com/exercism/exercism.io/issues/1867 the
exercise didn't adhere to the standard definition of the Hamming
distance which applies only to strings of the same length.

The following changes are made to fix the problem:

* The example solution is changed to respect the standard definition.

* Test cases for strings of differing lengths are removed.

This commit also contains the following additional changes:

* Some stylistic improvements for the test suite

* The name of the function is changed from 'hamming' to 'distance'
  resulting in a qualified name of 'hamming.distance'.

Fixes https://github.com/exercism/xpython/issues/115