Allows the caller to pass in argv to hl.main()

[jiawen]

No description provided.

[jiawen]

Any suggestions on how / where to put tests? A simple test would be to use something like SimplyPyGenerator and call it a few times with different values of -f. But then we'd need to verify what was written to disk.

[jiawen]

Hold off on merging - it occurs to me that one can implement hl.main() entirely in Python - there's even a place for it. Then the pybind11 code won't be nearly as messy, using embed().

[jiawen]

@alexreinking PTAL. Much cleaner I think.

[alexreinking]

Looks good with one nitpick

[alexreinking]

Argv arrays are meant to be NULL-pointer terminated. I don't think we rely on this today, but a very reasonable refactor to generate_filter_main might rely on it later.

[alexreinking]

Suggested change

	int result = Halide::Internal::generate_filter_main((int)mutable_argv.size(), mutable_argv.data(), PyGeneratorFactoryProvider());
	int result = Halide::Internal::generate_filter_main((int)mutable_argv.size() - 1, mutable_argv.data(), PyGeneratorFactoryProvider());

[alexreinking]

I fixed up the formatting in _generator_filter_main and tweaked names to make it clear that a Python list of arguments is getting translated into a compliant argv array.