Define reasonably stable C-ABI

[bwasti]

This project is almost entirely in TypeScript. Autograd, modules, loss functions, optimizers - all of it! The only part in C++ is bound exclusively through an ABI. That means anyone can roll their own backend and get all of shumai.

The ABI is quite simple, but not formalized. There are random _ prefixes and capitalizations. We should formalize it, publish it, and maintain it.

To emphasize the point: currently, you could compile this file as

g++ api.cc -std=c++11 --shared -o libflashlight_binding.so

and if that library (libflashlight_binding.so) ends up in the same directory as where you invoke shumai code, it'll use that instead of the default flashlight integration.