feat(library/standard/logic/connectives/if): add more general if_congr theorem

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/unifier): constraint priority in the unifier, and remove hack from if.lean

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(build): delete incorrect/old .d and .olean files, detect errors when generating .d files.

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