subsequences now use offsets into the shared_ptr seq_string

subsequences now use offsets into the shared_ptr seq_string
A subsequence created with the Sequence::subseq call will use
the creating sequences shared_ptr, and then just maintains
a seq_start and seq_count to indicate the start and number of bytes
in the subseq. (Yes this means that sometiems the c_str will let you
read more than you should actually be able to. But try not to depend
on that).