feat(library/standard/if): cleanup 'if-then-else' theorems

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/standard/bool_decidable): cleanup bool_decidable, and remove the artificial dependency to bit

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/standard/bool_decidable): cleanup bool_decidable, and remove the artificial dependency to bit