Make a line iterator about hex_math

[leftiness]

wip

• Code compiles. Existing tests pass.
• Need to write new tests for line::Iterator::next(). He also needs to be refactored into more functions?
• Merge #84 into this issue?
• Code is rough around the edges in general. Needs a second pass. It's too late to keep working tonight, though.

What I did:

• Remove Predicate trait. It turns out that you can use the existing iterator API without providing boxed closures.
• Change line::generic() into line::Iterator. He still infinitely iterates unless you filter him, but now I'm not the one handling enumeration, predicate application, etc. Stdlib does that.
• Make denumerate() function to reverse the effect of Iterator::enumerate()
• Alter Walls and Range predicates to be used with Iterator::scan().
• Range now returns None if index is greater than range. Previously, it was greater-or-equal. This boolean condition changed because the finer points of the logic changed when I converted from a loop to an iterator.

[leftiness]

Now that I've got this iterator stuff implemented, I should look at making other iterators.

Maybe Range/flood could use the same iterator with different predicates. Not sure.

Rotate and travel could maybe be iterators. They'd probably be a lot easier to write, but it also doesn't make as much sense. I mean.. maybe. Maybe you could provide arbitrary predicates to travel. Travel until you run out of move points maybe? Idk. Don't implement those yet. If I find a good use case, then do it. Otherwise it's just added complexity. Travel is already a very simple function, and it doesn't seem like he needs predicates.

Line of sight? I guess he could be an iterator. You could take from the iterator until you run out of vision distance or something? But... wouldn't you /always/ want to see as far as you can? There's no circumstance in which you would maybe want to break that loop /before/ you've seen everything that you can... Is there?

[leftiness]

Merged