r98: unpack a subset of sequences
added exact match; not working yet
r100: fixed a nasty bug caused by kstring_t

In the new kseq.h, we have "struct{size_t l,m}kstring_t", but in the kstring.h,
we have "struct{uint32_t l,m}kstring_t". In seq.c where kseq is instantiated,
kstring_t in kstring.h overrides kseq.h, which makes the "kstring_t" type
different and causes segfault.

In the old kseq.h, we have "struct{unsigned l,m}kstring_t", which is fine.