refactor(library/unifier): group flex_rigid case related methods in a functional object

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
perf(library/unifier): improve flex_rigid performance

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(kernel/converter): missing delay_check case

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>