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- new pybind11::base<> attribute to indicate a subclass relationship
- unified infrastructure for parsing variadic arguments in class_ and cpp_function
- use 'handle' and 'object' more consistently everywhere

Previously, pybind11 required classes using std::shared_ptr<> to derive
from std::enable_shared_from_this<> (or compilation failures would ensue).

Everything now also works for classes that don't do this, assuming that
some basic rules are followed (e.g. never passing "raw" pointers of
instances manged by shared pointers). The safer
std::enable_shared_from_this<> approach continues to be supported.

The cpp_function class accepts a variadic argument, which was formerly
processed twice -- once at registration time, and once in the dispatch
lambda function. This is not only unnecessarily slow but also leads to
code bloat since it adds to the object code generated for every bound
function. This change removes the second pass at dispatch time.

One noteworthy change of this commit is that default arguments are now
constructed (and converted to Python objects) right at declaration time.
Consider the following example:

py::class_<MyClass>("MyClass")
    .def("myFunction", py::arg("arg") = SomeType(123));

In this case, the change means that pybind11 must already be set up to
deal with values of the type 'SomeType', or an exception will be thrown.
Another change is that the "preview" of the default argument in the
function signature is generated using the __repr__ special method. If
it is not available in this type, the signature may not be very helpful,
i.e.:

|  myFunction(...)
|      Signature : (MyClass, arg : SomeType = <SomeType object at 0x101b7b080>) -> None

One workaround (other than defining SomeType.__repr__) is to specify the
human-readable preview of the default argument manually using the more
cumbersome arg_t notation:

py::class_<MyClass>("MyClass")
    .def("myFunction", py::arg_t<SomeType>("arg", SomeType(123), "SomeType(123)"));