Support cache expiration

[yysskk]

Example codes

 /// The expiration date is `.never`.
memoryCache.set(dog, for: .dog, expiration: .never)

 /// The expiration date is `.seconds("""10s""")`.
memoryCache.set(dog, for: .dog, expiration: .seconds(10))

 /// The expiration date is `.date("""TOMORROW""")`.
memoryCache.set(dog, for: .dog, expiration: .date(Date().addingTimeInterval(60 * 60 * 24)))

 /// Remove the cache of the specified key if it expired.
memoryCache.removeIfExpired(for: .dog)