RFC #17: Readonly interface fields

[ProCode-Software]

• a. Add a writable/write keyword that can be applied on fields, similar to readonly on structs. The downside is that is another keyword, when we already have readonly.
• b. All fields are writable by default, and readonly can be used to override.
• c. Interface fields can't be modified at all. Mutation requires a setter (e.g. setFoo()).
  Note that requiring fields to be writable makes the interface harder to implement. If a struct has a field readonly foo, it can't implement any interface that requires foo. Option B implicitly makes this too restrictive.