Iteration over SAM header lines and header expression filtering

[jmarshall]

A proposal to add API functions allowing user code to iterate over all header lines in order. Then with a way of referencing a particular generic header line established, we can add a sam_hdr_passes_filter() function to enable using filter expressions on SAM header lines.

Draft because the proposed basic iteration API (and everything else) is still in flux: the _line_ in the names can probably be removed, and I am considering changing to C++-esque begin/end terminology (which is an option for a SAM aux field iterator, and it would be good to use the same terminology for both) instead of first/next.

[jmarshall]

Draft because the proposed basic iteration API (and everything else) is still in flux: the line in the names can probably be removed

Updated to a more final version of the proposed API names. Still needs some test cases probably…

[jmarshall]

~~(CI failures are because Cirrus's catalina-base image contains an outdated and now-broken brew. See cirruslabs/osx-images#37; apparently Catalina is too old 🤣 and is deprecated on Cirrus.)~~