Add compile-time guard for CRTP hack.
Refactor bitstream classes.

Moved the iterator definition inside the respective bitstream classes for
better encapsulation.
Rename bitvector iterators.
Add EWAH sequence range.

Now that we can traverse EWAH bitstreams at the granularity of sequences, we
can efficiently implement bitwise operations without needing to decompress the
bitstream.

Relates #26.