Add "tseq" output & make available to Python

[nickzoic]

It'd be handy to be able to retrieve the sequence which minimap2 has matched with, to generate HGVS strings or etc.

You could reconstruct this from the long form CS string (see #1194) but it seems like it'd be handy to just retrieve the whole string in one go.

This PR adds an mm_gen_tseq C function to retrieve the sequence, and a tseq property on the python Alignment object.

I've called it "tseq" because it's called that in the write_cs_ds_or_MD function but it's not necessarily a great name.