Refactor Board & Piece

[LB--]

Back when I initially refactored this project, I ended up having to make Piece an inner class of Board because I didn't know you didn't need a complete type for references. As a result I designed everything to work under the assumption that Piece was internal to Board.

Unfortunately we now need to split them apart. In this PR my goals are to:

• Move Piece out of Board (done)
• Make Board and Piece not friends with each other (current)
• Implement Interactions to support both Castling and Promotion
• Check/checkmate
• Anything else as discussed below

Please look over the changes with each commit and comment on what you think needs to be changed.

@ResidentBiscuit I assigned you to this PR since we're letting Thumper relax for a while.

[Dyrand]

Are you going to do that before you commit the pull or after?

[LB--]

We're not going to merge this until I've completed all of the goals, I just wanted a way to allow for discussion as I worked.

[jaredready]

While you're digging around in here, would it be much to implement check/checkmate functionality? The code for movement and capturing looks pretty complicated and I wasn't able to figure out what was going on.

[LB--]

I thought I listed that on my goals, I've edited it in now.

What confuses you with movement and capturing?

[LB--]

I don't remember if this was ever finished. I guess it has to be now.

[jaredready]

Well it builds and runs. I looked it over and didn't see anything obviously missing. It's a lot cleaner approach than the previous.