Validate usage of types Java 8 defines as "value-based" to comply with their restrictions

Java 8 defines the concept of a "value-based class" and prohibits certain things being done with them; see for example Optional, or temporal types like Instant. We should enumerate these. Some of the things they want you not to do are things EP does already complain about in general (or we want it to), but I suspect we can get away with much stricter enforcement for this list of new value-based classes than we can in the general case.