Use codes::Instruction in emulator::CPU

[jeremyBanks]

Initially, the only logic we'll be using is decoding individual instructions -- not touching the ROM types.

This will still require a lot of change, and possibly some horrible giant match blocks until/unless Rust eventually adds impl for enum variants: rust-lang/rfcs#1450.

[jeremyBanks]

This is mostly done, but something is broken -- the logo tiles aren't being initialized.

[jeremyBanks]

Fixed. I'd swapped some flags when converting the code.