feat(library/standard/logic/connectives/if): add 'if-then-else' congruence theorem

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/standard/logic/classes): add 'by_contradiction' theorem for decidable propositions

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
chore(library/standard/logic/connectives/if): cleanup

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/Makefile.common): clean remove olean/d files recursively

[skip ci]