Indexing containers should not include trivia

[LeopoldArkham]

Indexing a container currently takes whitespace and comments into account. It shouldn't.

Thought: Perhaps we could have a wrapper type around usize called Exhaustive or Trivia that does include them when used as the index? People can just use nth() on the exhaustive iterator otherwise.

All the iterator code (in container.rs) may need a glance over actually; it's prototype quality and may be backwards.