Add RetroAchievements support

[nfp0]

Almost all bsnes and higan cores in Retroarch support RetroAchievements (even higan Accuracy and nSide Balanced, contrary to the info in the docs).

It would be great to have this excellent core also supporting RetroAchievements. Maybe code from the other bsnes/higan cores can be ported to this one to save time.

According to a contributor on the upstream repository, the missing implementation is this:

bsnes-hd/bsnes/target-libretro/libretro.cpp

Lines 1004 to 1014 in 87173e8

	// Currently, there is no safe/sensible way to use the memory interface without severe hackery.
	// Rely on higan to load and save SRAM until there is really compelling reason not to.
	RETRO_API void *retro_get_memory_data(unsigned id)
	{
	return nullptr;
	}
	RETRO_API size_t retro_get_memory_size(unsigned id)
	{
	return 0;
	}

Byuu says he won't do it himself but he'll be happy to accept a pull request if anyone implements this.
Is this a tall order, considering all previous higan/bsnes cores already implemented this?

I've also opened this issue here libretro/bsnes#27 on the libretro vanilla bsnes repository.

[DerKoun]

I have to defer this to libretro. Memory is a part of the emulator I have no idea about. So I'll close this as out of scope, as it is not a video feature. Sorry.

[nfp0]

Sure. It's understandable.