enum_: move most functionality to a non-template implementation

This commit addresses an inefficiency in how enums are created in
pybind11. Most of the enum_<> implementation is completely generic --
however, being a template class, it ended up instantiating vast amounts
of essentially identical code in larger projects with many enums.

This commit introduces a generic non-templated helper class that is
compatible with any kind of enumeration. enum_ then becomes a thin
wrapper around this new class.

The new enum_<> API is designed to be 100% compatible with the old one.

docs/changelog.rst

@@ -22,6 +22,11 @@ v2.3.0 (Not yet released)
 * Added support for write only properties.
   `#1144 <https://github.com/pybind/pybind11/pull/1144>`_.
 
+* Python type wrappers (``py::handle``, ``py::object``, etc.)
+  now support map Python's number protocol onto C++ arithmetic
+  operators such as ``operator+``, ``operator/=``, etc.
+  `#1511 <https://github.com/pybind/pybind11/pull/1511>`_.
+
 * A number of improvements related to enumerations:
 
    1. The ``enum_`` implementation was rewritten from scratch to reduce

tests/test_enum.py

@@ -153,4 +153,4 @@ def test_enum_to_int():
 def test_duplicate_enum_name():
     with pytest.raises(ValueError) as excinfo:
         m.register_bad_enum()
-    assert str(excinfo.value) == "Enum error - element with name: ONE already exists"
+    assert str(excinfo.value) == 'SimpleEnum: element "ONE" already exists!'

tests/test_pickling.py

@@ -34,3 +34,9 @@ def test_roundtrip_with_dict(cls_name):
     assert p2.value == p.value
     assert p2.extra == p.extra
     assert p2.dynamic == p.dynamic
+
+
+def test_enum_pickle():
+    from pybind11_tests import enums as e
+    data = pickle.dumps(e.EOne, 2)
+    assert e.EOne == pickle.loads(data)