Better support for const buffers

[nedrebo]

The buffer_info class takes a void *. I propose to support const void *.

• Removes need for const_cast when you have a const T * (only possible if you know memory is mutable)
• Read only access of mutable memory will be supported by using std::as_const or a safe "to const" const_cast.

[nedrebo]

Related PR: #1466

I think a const "overload", as proposed here, is better. With two parameters, as proposed in #1466, you need to keep them in sync.That is a source of errors. Also the buffer_info signature does not guarantee that memory will not be mutated even when you pass in a readonly=true. Passing in const memory after const_cast it is still scary with readonly=true.

[wjakob]

PR #1466 was just merged.

[nedrebo]

Fixed in #1466 by adding this constructor.

pybind11/include/pybind11/buffer_info.h

Lines 50 to 52 in dc65d66

	template <typename T>
	buffer_info(const T *ptr, ssize_t size, bool readonly=true)
	: buffer_info(const_cast<T*>(ptr), sizeof(T), format_descriptor<T>::format(), size, readonly) { }