Support keyword arguments and generalized unpacking in C++

A Python function can be called with the syntax:
```python
foo(a1, a2, *args, ka=1, kb=2, **kwargs)
```
This commit adds support for the equivalent syntax in C++:
```c++
foo(a1, a2, *args, "ka"_a=1, "kb"_a=2, **kwargs)
```

In addition, generalized unpacking is implemented, as per PEP 448,
which allows calls with multiple * and ** unpacking:
```python
bar(*args1, 99, *args2, 101, **kwargs1, kz=200, **kwargs2)
```
and
```c++
bar(*args1, 99, *args2, 101, **kwargs1, "kz"_a=200, **kwargs2)
```

tests/test_callbacks.py

@@ -27,6 +27,41 @@ def test_callbacks():
     assert f(number=43) == 44
 
 
+def test_keyword_args_and_generalized_unpacking():
+    from pybind11_tests import (test_tuple_unpacking, test_dict_unpacking, test_keyword_args,
+                                test_unpacking_and_keywords1, test_unpacking_and_keywords2,
+                                test_unpacking_error1, test_unpacking_error2,
+                                test_arg_conversion_error1, test_arg_conversion_error2)
+
+    def f(*args, **kwargs):
+        return args, kwargs
+
+    assert test_tuple_unpacking(f) == (("positional", 1, 2, 3, 4, 5, 6), {})
+    assert test_dict_unpacking(f) == (("positional", 1), {"key": "value", "a": 1, "b": 2})
+    assert test_keyword_args(f) == ((), {"x": 10, "y": 20})
+    assert test_unpacking_and_keywords1(f) == ((1, 2), {"c": 3, "d": 4})
+    assert test_unpacking_and_keywords2(f) == (
+        ("positional", 1, 2, 3, 4, 5),
+        {"key": "value", "a": 1, "b": 2, "c": 3, "d": 4, "e": 5}
+    )
+
+    with pytest.raises(TypeError) as excinfo:
+        test_unpacking_error1(f)
+    assert "Got multiple values for keyword argument" in str(excinfo.value)
+
+    with pytest.raises(TypeError) as excinfo:
+        test_unpacking_error2(f)
+    assert "Got multiple values for keyword argument" in str(excinfo.value)
+
+    with pytest.raises(RuntimeError) as excinfo:
+        test_arg_conversion_error1(f)
+    assert "Unable to convert call argument" in str(excinfo.value)
+
+    with pytest.raises(RuntimeError) as excinfo:
+        test_arg_conversion_error2(f)
+    assert "Unable to convert call argument" in str(excinfo.value)
+
+
 def test_lambda_closure_cleanup():
     from pybind11_tests import test_cleanup, payload_cstats