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This is required since format descriptors for string types that
were using PYBIND11_DESCR were causing problems on C++14 on Linux.

Although this is technically a breaking change, it shouldn't cause
problems since the only use of format strings is passing them to
buffer_info constructor which expects std::string.

Note: for non-structured types, the const char * value is still
accessible via ::value for compatibility purpose.

Add handle::repr() method

py::dtype, buffer protocol improvements, structured types support