perf(library/unifier): improve check_imitation performance

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/explicit): allow 'as-is', 'explicit' and 'implicit' annotations to be saved in .olean files

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/explicit): allow 'as-is', 'explicit' and 'implicit' annotations to be saved in .olean files