Add API functions to parse and format SEQ and QUAL fields

[jmarshall]

This adds functions to the public API to pack and unpack the SEQ and QUAL fields individually, enabling third-party code to take advantage of the optimised and SIMD-optimized implementations of this functionality that HTSlib provides.

The format ones were the motivation for this; in particular they will be immediately useful for pysam. The parse ones are perhaps of less widespread use (at least in their current form) as usually if writing to a bam1_t there’ll need to be some memory reallocating going on too. But I think pysam would benefit from accessing HTSlib’s implementations of these too.

[daviesrob]

This mostly looks OK to me. It might be good to mention in the documentation exactly how big the destination buffer needs to be, especially for bam_format_seq() and bam_format_qual() where it's a bit less obvious. Also exactly what's going on in sam_format_qual() and sam_parse_qual() - that 33 is added to / subtracted from the values to convert them to / from printable ASCII; and that the sam_parse_qual() error state means that conversion failed because one of the input values was too small.