Reimplement gap filling with right extensions on the reverse complement

[tmaklin]

SBWT API allows cheap right extensions but we currently do expensive left extension to fill gaps. By reverse complementing the neighborhood around the gap, the filling could take advantage of right extension instead.

[tmaklin]

This seems like it won't be a drop-in replacement and might cause differences in the results. Additionally, the tests break unless the default BuildOpts are changed to include reverse complements.

Therefore prob best to leave for a more major update that breaks backwards compatibility.

Some work started in add-right-extensions