Add support for positional args with args/kwargs

This commit rewrites the function dispatcher code to support mixing
regular arguments with py::args/py::kwargs arguments.  It also
simplifies the argument loader noticeably as it no longer has to worry
about args/kwargs: all of that is now sorted out in the dispatcher,
which now simply appends a tuple/dict if the function takes
py::args/py::kwargs, then passes all the arguments in a vector.

When the argument loader hit a py::args or py::kwargs, it doesn't do
anything special: it just calls the appropriate type_caster just like it
does for any other argument (thus removing the previous special cases
for args/kwargs).

Switching to passing arguments in a single std::vector instead of a pair
of tuples also makes things simpler, both in the dispatch and the
argument_loader: since this argument list is strictly pybind-internal
(i.e. it never goes to Python) we have no particular reason to use a
Python tuple here.

Some (intentional) restrictions:
- you may not bind a function that has args/kwargs somewhere other than
  the end (this somewhat matches Python, and keeps the dispatch code a
  little cleaner by being able to not worry about where to inject the
  args/kwargs in the argument list).
- If you specify an argument both positionally and via a keyword
  argument, you get a TypeError alerting you to this (as you do in
  Python).

tests/test_kwargs_and_defaults.py

@@ -55,3 +55,52 @@ def test_arg_and_kwargs():
     args = 'a1', 'a2'
     kwargs = dict(arg3='a3', arg4=4)
     assert args_kwargs_function(*args, **kwargs) == (args, kwargs)
+
+
+def test_mixed_args_and_kwargs(msg):
+    from pybind11_tests import (mixed_plus_args, mixed_plus_kwargs, mixed_plus_args_kwargs,
+                                mixed_plus_args_kwargs_defaults)
+    mpa = mixed_plus_args
+    mpk = mixed_plus_kwargs
+    mpak = mixed_plus_args_kwargs
+    mpakd = mixed_plus_args_kwargs_defaults
+
+    assert mpa(1, 2.5, 4, 99.5, None) == (1, 2.5, (4, 99.5, None))
+    assert mpa(1, 2.5) == (1, 2.5, ())
+    with pytest.raises(TypeError) as excinfo:
+        assert mpa(1)
+    assert msg(excinfo.value) == """
+        mixed_plus_args(): incompatible function arguments. The following argument types are supported:
+            1. (arg0: int, arg1: float, *args) -> tuple
+
+        Invoked with: 1
+    """  # noqa: E501
+    with pytest.raises(TypeError) as excinfo:
+        assert mpa()
+    assert msg(excinfo.value) == """
+        mixed_plus_args(): incompatible function arguments. The following argument types are supported:
+            1. (arg0: int, arg1: float, *args) -> tuple
+
+        Invoked with:
+    """  # noqa: E501
+
+    assert mpk(-2, 3.5, pi=3.14159, e=2.71828) == (-2, 3.5, {'e': 2.71828, 'pi': 3.14159})
+    assert mpak(7, 7.7, 7.77, 7.777, 7.7777, minusseven=-7) == (
+        7, 7.7, (7.77, 7.777, 7.7777), {'minusseven': -7})
+    assert mpakd() == (1, 3.14159, (), {})
+    assert mpakd(3) == (3, 3.14159, (), {})
+    assert mpakd(j=2.71828) == (1, 2.71828, (), {})
+    assert mpakd(k=42) == (1, 3.14159, (), {'k': 42})
+    assert mpakd(1, 1, 2, 3, 5, 8, then=13, followedby=21) == (
+        1, 1, (2, 3, 5, 8), {'then': 13, 'followedby': 21})
+    # Arguments specified both positionally and via kwargs is an error:
+    with pytest.raises(TypeError) as excinfo:
+        assert mpakd(1, i=1)
+    assert msg(excinfo.value) == """
+        mixed_plus_args_kwargs_defaults(): got multiple values for argument 'i'
+    """
+    with pytest.raises(TypeError) as excinfo:
+        assert mpakd(1, 2, j=1)
+    assert msg(excinfo.value) == """
+        mixed_plus_args_kwargs_defaults(): got multiple values for argument 'j'
+    """