Try to think through EmptyLook support

[ethanpailes]

I've just been ignoring this, but supporting emptylooks seems like a nice thing to do. It would be nice to actually be fully backwards comptabable with standard regex.

[ethanpailes]

If I manage to support empty look stuff I could stream all the rust source down from crates.io and pull out the regular expressions to check for the applicability of my various optimizations.

[ethanpailes]

So it seems like the VM side of things is fairly doable. For compilation and tset-intersection and whatnot things will be harder

Tset Intersection

Rather than just answer the questions "can this expression start with this char?" and "what chars can this expression start with?" I will need to deal with the potential position of the char and its predecessor.

Perhaps I can just conservetivly say that all test chars are simultaniously at the front, the middle, and the back of the input.

I think I can drop any emptylooks at the beginning of a concatination, and otherwise treat them as a . to be on the conservative side.

Expression Intersection

TODO

Compilation

I think emptylooks get compiled as skip 0 unless they are in branch position in which case they get compiled normally. I think this change will nececcitate doing real skip fusion rather than leaning on the parser to clump literals together (#9).