Iterate through sequences as they are read.
Iterate through sequences as they are read.

Add fasta_sequence_iterator which returns bases as they are read from
the input file, and change parse_fasta_format to return this iterator
instead of the entire sequence.  Update seq_iter to work with this iterator.
This removes the need to keep the entire sequence in memory.

daviesrob/qu-mfeindex,daviesrob/qu-mfeindex