Automate check_parser_state in BoxIter loops

[baumanj]

By convention, the check_parser_state macro is called at the end of while loops for iterating over BMFFBox's sub boxes with BoxIter like so:

let mut iter = src.box_iter();
while let Some(mut b) = iter.next_box()? {
    // read or skip all the contents of `b`
    check_parser_state!(b.content);
}

This helps to surface the situation of failing to read boxes in their entirety either due to logic errors or malformed input. However, it can be easy to forget to add the check_parser_state macro in the appropriate place, so some form of automation would improve the robustness of the codebase.

[baumanj]

See #239 (comment) for some thoughts on potential solutions.

[baumanj]

Now that generic associated types (GATs) have landed in stable, that's probably a preferable approach. The parser state check can probably be inserted in the Iterator's next method.