[META] Project restructure

[dannyhammer]

Description:
Since the merge of #73, the code is in need of better organization. Either use a cargo workspace or just break up the project into individual modules. Core types, board representation, move generation, input handling, engine, and search are all good candidates for modules.

Relevant Link(s):

• https://doc.rust-lang.org/book/ch14-03-cargo-workspaces.html

Requirements:

• Determine whether to use a workspace or individual modules
• Use build scripts, if possible, to precompile things like move tables
• Split up code into crates/modules