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.py

@@ -220,3 +220,19 @@ def test_print(capture):
         if debug_enabled else
         "arguments to Python object (compile in debug mode for details)"
     )
+
+
+def test_hash():
+    class Hashable(object):
+        def __init__(self, value):
+            self.value = value
+
+        def __hash__(self):
+            return self.value
+
+    class Unhashable(object):
+        __hash__ = None
+
+    assert m.hash_function(Hashable(42)) == 42
+    with pytest.raises(TypeError):
+        m.hash_function(Unhashable())