Expose all retro_* callbacks in RetroCore.

[InquisitiveCoder]

Exposes all retro_* callbacks in RetroCore. This is necessary to future-proof the crate's API, since environment commands can be specific to certain callbacks. In my opinion it also makes the crate easier to learn since it'll have a close mapping to the C API.

Also makes some slight alterations to the existing RetroCore methods, like removing the redundant size parameter and using associated types instead of just u32 for user-defined enums.

[ghost]

@InquisitiveCoder let me know when you get this rebased and I'll begin reviewing 👍🏼

[InquisitiveCoder]

@InquisitiveCoder let me know when you get this rebased and I'll begin reviewing 👍🏼

Good to go.

[ghost]

Make sure your markdown links work. And, if they are links to code constructs then they are of the form:

[`reference_here`]

Once the comments below are addressed (either implemented, or tell me to kick rocks 😄) I'll merge this. Thanks!