`pdfplumber.utils` functions should take `Iterable` and not `List` arguments

[dhdaines]

Currently, objects_to_bbox and friends require a list as input, but there's no reason they couldn't work equally well with generator expressions, iterators, or anything else iterable.

Well, actually there is a reason, which is that they are written such that they iterate repeatedly over their input for each attribute (and thus can't reuse the same iterable), but this doesn't have to be the case, for instance, merge_bboxes could be written as:

def merge_bboxes(bboxes: Iterable[T_bbox]) -> T_bbox:
    x0, top, x1, bottom = zip(*bboxes)
    return (min(x0), min(top), max(x1), max(bottom))

It's also a bit faster (note that there is no need to wrap the map in a list):

In [46]: %timeit pdfplumber.utils.merge_bboxes(list(map(bbox_getter, page.chars)))
464 µs ± 1.48 µs per loop (mean ± std. dev. of 7 runs, 1,000 loops each)
In [49]: %timeit merge_bboxes(map(bbox_getter, page.chars))
409 µs ± 1.18 µs per loop (mean ± std. dev. of 7 runs, 1,000 loops each)

I can supply a PR :)

[dhdaines]

See #946