"Public" libres API changes

[pinkwah]

Move all internal structs in libres to public header files. This will help us instantly remove most getters/setters used by C code.

Some setters need to allocate strings osv. Replace char* with std::string.

In Python cwrap code, consider using ctypes.Struct to define the struct in Python, and then get the struct members directly, rather than using getters.

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Use new / delete rather than malloc / free in _alloc and _free functions.

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If possible, do the jump directly to pybind11-backed C++ classes.