Separate REXPaint parser to its own crate

[GGalizzi]

Should the rex parser be separated into its own isolated crate? Given the modular nature of bracket-lib I think it makes sense. XpFile/Layer/Cell can easily be separated.

And then there's XpColor which, if this issue is accepted as desired, might also make more sense to be in its own crate alongside the other Xp*

Thoughts?

[thebracket]

Not a bad idea, and shouldn't be too hard to implement. I had it as part of terminal before because it was heavily derived from https://gitlab.com/medusacle/rs-rexpaint (the licenses are compatible and the original is credited), but with a few tweaks. Thinking about it, since it's already gated by a feature flag this should be a pretty easy one. I'll see what I can come up with.

[thebracket]

I'll work on this in the rex-crate branch: https://github.com/amethyst/bracket-lib/tree/rex-crate

[thebracket]

I don't have time to finish integration testing today, so I'll try and get to that soon. I had to also break out the embedding support into a crate, so it could be used by both rex and bracket-terminal - but I kinda like that, because it's been stable/unchanged for a long time, and cleans up the macro mess in bracket-terminal/src/lib.rs.

I don't think this will break anything important, but I'm not going to merge it until I've checked. :-)

[thebracket]

Ok, I have it running the Hands-on Rust examples (including the wasm embedded version) and the Roguelike Tutorial. So I think I'm ready to merge this.