Add ANSI parser, and cursor support.

[thejpster]

phew.

Like the Monotron, we cheat with the cursor. Instead of making the BIOS support a hardware cursor, we fake it in software.

1. To enable the cursor, cache the glyph currently at the cursor position and replace it with a _
2. To disable the cursor, put the cached value back
3. Ensure you temporarily disable the cursor anytime you write to the screen, or really do anything with the screen
4. Realise you need to recursively enable and disable the cursor, because writing to the screen now goes through the ANSI state machine and that may then emit a second, recursive, write to the screen. Oof.