Access C++ hash functions from Python and vice versa (#1034)

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.

tests/test_pytypes.cpp

@@ -261,4 +261,6 @@ TEST_SUBMODULE(pytypes, m) {
     });
 
     m.def("print_failure", []() { py::print(42, UnregisteredType()); });
+
+    m.def("hash_function", [](py::object obj) { return py::hash(obj); });
 }