Allow definition of a load NIF callback

[brodeuralexis]

It is sometimes necessary for a NIF to have some global initialization logic in its load callback, like we already have for atom and resource registration.

This commit inlines the fine::__private__::load function in the FINE_INIT macro to delay template instantiation of a new fine::__private__::OnLoad struct that can be overriden by a user using the FINE_LOAD(env) macro to provide customized initialization logic, while still registering atoms and resources.

[jonatanklosko]

Hey @brodeuralexis! I dropped a comment regarding the API :)

That said, I wonder if there is a logic that the user would put on load, which couldn't be a typical initialization. In the docs example, couldn't they could as well do this:

static ThreadPool s_pool = ThreadPool(std::thread::hardware_concurrency());

And if that's the case, I would probably hold off with extra APIs, until there is a use case.

[jonatanklosko]

FINE_NIF -> FINE_INIT. Also I would skip the explanation, since it's just implementation details.

Suggested change

	Callbacks MUST be used before `FINE_NIF`, since the formers use template
	specializations that are instantiated by the latter. If the order is reverse,
	there is no guarantee that `FINE_NIF` will actually invoke the callbacks.
	Callbacks MUST be used before `FINE_INIT`, otherwise there is no guarantee
	that `FINE_NIF` will actually invoke the callbacks.

[jonatanklosko]

I think macros should be self-contained, in the sense that calling FOO(bar) generates fully valid code, otherwise the syntax can be confusing (and I expect errors too). For consistency we also enforce semicolons after FINE_* registrations by adding static_assert(true, "require a semicolon after the macro") at the end of the generated code.

So, even though it's slightly less convenient, I would rather do this:

void load(ErlNifEnv* caller_env, void** priv_data, ERL_NIF_TERM load_info) {
  // ...
}

FINE_LOAD(load);

We likely should expose all the arguments that the erlang callback has.