SPM support for Core

[szotp]

Added basic support for Swift Package Manager. SPM can handle C libraries, and Swift supports binding to them, which means someone else can implement a wrapper on top of it.

I had to make some adjustments in headers:

• Unnecessary circular dependency of display.h on gb.h
• apu.h using GB_ENUM without including save_state.h
• save_state.h using GB_gameboy_t without including model.h

[szotp]

I tried to run https://pub.dev/packages/ffigen and it found further errors with missing includes.

Pushed fixes for those. Maybe it would make sense to add

#include <stdbool.h>
#include <stdint.h>
#include <stddef.h>

into defs.h and only include one file in other places?

[LIJI32]

I have several issues and comments:

1. Header cleanup is good, can be merged pretty much as is.
2. I'm rather worried about adding SPM support to the mainstream repo. I don't use Swift and I don't know how SPM works, so I couldn't possibly provide official support to it. Could this be done in a different repo with SameBoy being a subrepo?
3. The "ideal" way of using SameBoy as a library is using make lib (potentially with CONF=release and potentially extra CFLAGS/LDFLAGS/SYSROOT for cross-compilation if needed) and using the generated a/o/dylib file together with the post-processed headers. This makes sure SameBoy is built with LTO and the optimal optimization flags, but I worry the most about having GB_INTERNAL defined globally like you do here – if I understand SPM correctly, this would expose internal APIs to the Swift side, which is quite problematic.