Add ansi parser

[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.

[thejpster]

Still todo, test cases for:

• 'F' => Cursor previous line
• 'G' => Cursor horizontal absolute
• 'H' | 'f' => Cursor Position (or Horizontal Vertical Position)
• 'J' => Erase in Display (needs implementing too)
• 'K' => Erase in line (needs implementing too)

Also I should add a "cat" program so we can show off our new ANSI output with test files on disk.