Refactor `_find_chain_item`

`get_label_item` constructs a dict from all the labels that could potentially be in the call chain and then looks up the label. This simplifies the look-up but we could probably refactor it to bail early once we've found it instead. Need to profile, but I feel like we can get a little bit of a speed up here, especially for larger projects.

We could also get rid of `strip_type` by adding a property to `FortranVariable` to return it, and use that in `FortranVariable.full_type` too.