When comparing two objects and the appropriate rich comparison
method doesn't exist, the `__cmp__` method should be tried. For
example, if `a == b` is executed and neither `a` or `b` have
the appropriate rich comparison methods, then the equivalent
of `a.__cmp__(b) == 0` should be tried.

Lots of tests for both 3-way comparison and rich comparison were
added.  The comparison implementation in the runtime should be
fully covered now.

Python gaurantees that its builtin sort for mutable sequences is stable.
For more details, see note (9) in:

  * https://docs.python.org/2/library/stdtypes.html#mutable-sequence-types

I couldn't come up with a decent way to test this.

Implement the `cmp` as defined here:

* https://docs.python.org/2/library/functions.html#cmp

Implementing this builtin required implementing a fair
amount of supporting logic to do 3-way comparisons. The
3-way comparison implementation in CPython is somewhat
complex, thus there may be some cases that still need
support. This is a good start, though.

Pull values from Go syscall package.