Standalone C++ bindings

[TheBlueMatt]

Moved from lightningdevkit/rust-lightning#677

Discussed offline. In this variation, the C++ class would be LDKObject. Since the copy constructor would be deleted, any object passed to rust_fn would need to be an rvalue; it would be a compilation error to pass the object as an lvalue. Instead, the user would need to explicitly std::move the lvalue in order to pass it to rust_fn as an rvalue, which would trigger the move constructor.

Agreed to open an issue and prioritize based on user need for the C++ header.

Originally posted by @jkczyz in lightningdevkit/rust-lightning#618 (comment)