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.

Previously Go typedefs for simple types (e.g. `type myInt int`) would return the underlying base type (e.g. `int`) from __native__. They now correctly convert to the typedef when used in a Go context.

* Start implement string slicing
* implement unicode slicing

* Implement print_function future feature

This allows the import statement `from __future__ import print_function` to
work, by setting a flag on the parser to treat print statement/keyword as a
syntax error.

* Implemented via descriptors instead of overriding GetAttribute

* Implement os.chdir, os.getcwd.
* Implement os.path.abspath. Add _AssertEqual() to check not only values but also types.

The link was interpreted as local and did link to https://github.com/google/grumpy/blob/master/jyni.org instead of http://jyni.org

Implement the `any` as defined here:

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

Note that a new helper function `seqFindFirst` was added that
is used to implement `builtinAll`, `builtinAny`, and `seqContains`.

Implement the `all` as defined here:

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

This patch finishes implementing support for unary minus.
The slot and core dispatch functions were already there,
but the individual numeric slot implementations were missing.

* Implement os.path.dirname.
* Implement os.path.exists, os.path.isdir, os.path.isfile.