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It used to pass anyway because the expected output was identical
to the Python set.

- Conversion warning on clang: 'long' to 'size_t'
- Unused variable warning on MSVC

Sergey Lyskov pointed out that the trampoline mechanism used to override
virtual methods from within Python caused unnecessary overheads when
instantiating the original (i.e. non-extended) class.

This commit removes this inefficiency, but some syntax changes were
needed to achieve this. Projects using this features will need to make a
few changes:

In particular, the example below shows the old syntax to instantiate a
class with a trampoline:

class_<TrampolineClass>("MyClass")
    .alias<MyClass>()
    ....

This is what should be used now:

class_<MyClass, std::unique_ptr<MyClass, TrampolineClass>("MyClass")
    ....

Importantly, the trampoline class is now specified as the *third*
argument to the class_ template, and the alias<..>() call is gone. The
second argument with the unique pointer is simply the default holder
type used by pybind11.

args was derived from list, but cpp_function::dispatcher sends a tuple to it->impl (line #346 and #392 in pybind11.h).  As a result args::size() and args::operator[] don't work at all.  On my mac args::size() returns -1.  Making args a subclass of tuple fixes it.

Adding documentation for value_error. Fixing various formatting issues. Removing redundant binding for C++ style methods. Adding bindings for iterator and slicing protocol. Extending examples.

Adding Python-style modifiers for vector. Wrapping has_insertion_operator_implementation in _MSC_VER ifdef’s. Adding ‘!=0’ instead of bool cast’s.

This somewhat heavyweight solution will avoid size_t/long long/long/int
mismatches on various platforms once and for all. The previous template
overloads could e.g. not handle size_t on Darwin.

One gotcha: the 'format_descriptor<T>::value()' syntax changed to just
'format_descriptor<T>::value'