Check that RBS strength is conserved while attempting synonymous swaps.
Move is_rbs_strength_conserved() into rbs_calc_util and implement refactor_checker.check_rbs_conserved
Tweak to iterating through synonymous codons whe making variants so that the current codon is preferentially expanded first, so that if it's part of an 'adequate' solution first (e.g. conserves RBS and fits), then we'll use it.