fix(library/unifier): potentially changing flag from l_false ==> l_undef

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
perf(kernel/justification): make sure depends_on doesn't get 'lost' in justification objects with a lot of shared objects

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(kernel/metavars): remove inefficient 'occurs' method from substitution object

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