Basic tap parser.

This is a super-basic tap parser for overlord. I still don't know what I'm doing in rust, so I'm sure this could be greatly improved.

This needs a lot of work, so I'm not sure if it's worth saving, or doing over from scratch. I just wanted to get this submitted because I might be very busy over the next few weeks. Amongst the missing things:
* Leading whitespace - Should be ignored and looped into the previous comment. Not entirely sure the best way to do this because rust seems to collapse whitespace in strings?
* TAP Version parsing - pretty easy to do, just not sure if we want to do it.
* Properly handling SKIP/TODO directives.