Rewrite panda_expect.py

[AndrewFasano]

panda_expect.py is about 500 lines of hacks that reinvent existing libraries. I think the functionality we need largely exists in the following two libraries:

• pexpect for sending/receiving lines
• urwid for parsing ANSI escape codes

I'm particularly interested in urwid since I wrote the (terrible) ansi-parsing code that's currently used in panda_expect and imagine we'd get some performance + reliability improvements if we folded that in.