Simplest possible sort impl

This ist the simplest possible sort implementation I found (the one in libc++ is too optimized). 
It allows to at least try the sort interface.

Test using default comparison are provided.

TODO:
- test using iterators, custom comparisons, and projections.
- test file needs cleaning.
What's wrong with it? You could forward-declare the sort global, and then just say "`sort`". I personally see no need.