[kmseqfile] refactor strfseq -> seqfile_format_seq
[kmseqfile] add seqfile iterator macros

These macros setup serial or parallelised iterator loops that loop thru
a fast[aq] file, and write the per-sequence loop yourself. Less overhead
and more flexibility than the seqfile_iter_X function, that work with
function pointers. These haven't been removed yet though, not sure what
I'll do there.
[benchmarks] add seqfile iterator macro benchmarks
[kmutil] implement stringify macro

Turns macro args into a string