Move core initialization to load_game for better ergonomics

[InquisitiveCoder]

Addesses Issue #3: Move RetroCore instantiation to load_game for better ergonomics. Below is a summary of the changes:

• retro_init and retro_deinit are now no-ops.
• Removed RetroCore::init.
• RetroLoadGameResult contains an additional field for the RetroCore instance.
• RetroCore::load_game is now a function.
• Removed RetroInstance::on_init and on_deinit. The code has been moved to on_load_game and on_unload_game respectively.

With these changes, users of the crate return a fully initialized instance from RetroCore::load_game rather than return a partially initialized instance from init and updating it in load_game.

Since this isn't a backwards-compatible change, I've updated the minor digit of the version number.

[ghost]

Thanks for the PR! I have a few notes, mostly questions around libretro lifecycle that I'd like to hash out before merging.

[ghost]

Thanks for submitting this PR! I've reverted the change to the version number, as I'll bump that when the time comes for the next release. Feel free to submit more as you see fit!