double deletion skips singly lethal deletions

Assuming that singly lethal deletions give lethal double deletions
improves solves times. I observed speedups of up to 50%.

It also passes solver arguments on correctly.

This fixes #101
Merge pull request #106 from aebrahim/double_deletion_update

double deletion skips singly lethal deletions

This adds back in a feature previously found in double deletion. The new
double deletion code is now more or less at feature parity with the
older implementation, with the one exception that it does not accept
precomputed single deletions. However, these deletions can now be
computed very quickly (much faster than the double deletions will take).