Add an expermental alternative cpp backend

[sonder-joker]

A implementation of cpp (need c++17). Wish to get a review! The generated cpp.h pass the clang-tidy check.

[sonder-joker]

• I wish to add namespace in different module #10 . So I wish if you can help me to finish it!
• Also I wish if you can help me check the layout of struct/union in generate file. I don't sure I have make some stupid mistake.
• Some of inspired by wasi-libc's tool but it have abi change in wasi-crypto so I make a simple cpp backend.

[sonder-joker]

For current cpp backend I will:

• [x] Support return result as a Result(rust) like style as return value(also use std::tuple)
• [ ] Generate different namespace for each module
• [ ] Adopt LLVM style's code
• [ ] Support generate multiple files for each module (may not put in thie pull request).
• [ ] Check the layout with static_assert