Use struct instead of class for core structures

[aaronfranke]

This PR does three things:

• Change class to struct for the core data structures and adjusts the bindings generator to handle this.
• Add _NO_DISCARD_ to core data structures to match the engine.
• Remove _native_ptr from core data structures to match the engine (it's just a void pointer cast).

Why change class to struct? Here are several reasons:

• It matches what the core engine uses.
• struct's default visibility is public, which all members are now that _native_ptr has been removed.
• class and struct are actually different things on MSVC. With godot-cpp's types defined with class, any forward declarations copied from an engine module like struct Vector2; will not work correctly on MSVC. This is actually something that I discovered after starting work on this PR, and it makes the case for it stronger.
• It sets the expectation for users that this is a data structure. This point is purely cosmetic.

[akien-mga]

Thanks!