Wish list: aux tag iterator API

[jkbonfield]

(Put here simply so we don't forget it.)

The (now merged) Samtools PR samtools/samtools#516 copies over the aux_type2size and skip_aux functions as it has a mechanism of iterating through the tag list.

Something akin to these functions ought to be public, although perhaps not these explicit interfaces. I think a more iterator focused approach would work. Eg in a similar fashion to kstring initialisations:

bam_tag_iter_t it = BAM_TAG_ITER_INITIALIZE;  // or bam_tag_iter_initialize(&it) if we want it opaque.
uint8_t *tag;
while ((tag = bam_aux_next(b, &it))) {
    // do stuff
}

The tag initialise function just nulls the contents. The bam_aux_next function skips to the next tag, or identifies the first one if the pointer hasn't been set. The BAM data end pointer from skip_aux can be internalised into the data type and hidden from the user. Alternatively we just expose skip_aux as it is now and have an aux_end function to avoid the mess of grubbing through b->data + b->l_data internals.

aux_type2size may be useful as-is, as we already have search functions so being able to return how big it is would be helpful for more than this case. However it could be renamed perhaps - maybe bam_aux_size.