Add bespoke classes for accessing SEQ and QUAL

[jmarshall]

Inspired by the request in #1346, “Faster Single-Base Access for Long Reads”:—

Instead of accessing SEQ (and eventually QUAL) via a str property, add classes that implement __getitem__, __str__, etc directly from the internal BAM representation. These classes can then be used to access subsequences and do other useful things that you would do with a str:

aln.query_bases[4]
aln.query_bases[3:12]
print(f"Sequence is {aln.query_bases}")
…etc…

without decoding the unrequested parts of the sequence data.

This demonstrates the idea. We will likely rename this and position it to become the principal way to access SEQ, deprecating query_sequence, and similarly for base qualities.