Include overlap_size in conflicting_pair_finder as it will be used in the fixer.
Implement handling for overlaps of size 4 that are not fixed by
synonymous coding.

This takes the last 3 codons of the upstream feature and copies them to
the end of the upstream feature, and then pushes the downstream feature
over 3, leaving a single overlap. It then checks that RBS is conserved
and continues on its way.