Iterator over sequence range
Iterator over sequence range

Use new sequence range iterator to implement Sequence::string() and TwoBit::base_frequencies().
This resulted in a 4 to 5x improvement on TwoBit::base_frequencies(). This is now about 2x slower than C
which is not bad given Rust may not be as optimized as it can be.

andrelmartins/TwoBit,andrelmartins/TwoBit,andrelmartins/TwoBit