This commit revamps the object_api class so that it maps most C++
operators to their Python analogs. This makes it possible to, e.g.
perform arithmetic using a py::int_ or py::array.
There are two separate additions:
1. `py::hash(obj)` is equivalent to the Python `hash(obj)`.
2. `.def(hash(py::self))` registers the hash function defined by
`std::hash<T>` as the Python hash function.
