Allow simply distinguishing enums from option sets

[Chris Hanson]

C enums are used for a couple of different purposes. They're used for traditional pick-one-of-many enumerations that are used in e.g. switch/case constructs. However, they're also used to impart additional type system support to bit fields where several items can be or'd together.

To improve IDE support and Swift interoperability, Apple added C++ enum "base type" support to Objective-C, and also added two macros that the IDE and the Swift C importer can look at to determine the intent of an enum: CF_ENUM() and CF_OPTIONS(). (The more commonly used NS_* variants are just wrappers for these.)

It could be useful for CFFI to have some comparable way of annotating C enumerations for the same purpose. For example, an API that takes an imported C enum marked as an option set could accept a list like (:flag1 :flag2) to represent the bitwise OR of values from the enum, while disallowing the same for a C enum not representing an option set.