Reconsider re-export strategy

[joshlf]

Currently, we re-export many items from the crate root, notably including all items from the byteorder module. Historically, we haven't had a consistent philosophy about this. Before 0.8 is a good time to reconsider this strategy. We should:

• Come up with a consistent strategy about when to re-export
• For any exports that currently exist that we'd like to remove, mark them as #[deprecated] for 0.8, and plan to remove them in 0.9
• While we're at it, this may be a good time to consider a more general module refactoring (#252)