Allow for a type to be defined as readonly

[jimbxb]

A readonly type would be a type that, once a value of this type is defined, cannot be modified using any of the usual mutators defined automatically for a type.

The implementation of this should be as simple as only creating the getters (not mutators) for a readonly type. However, it might just be better to make the mutators private, as this would allow modifications to a value inside the type, but not outside.

A type could be defined to be readonly using the type modifiers we have currently for unique types. A la

type {readonly} tuple(A, B) { tuple(fst:A, snd:B) }

[jimbxb]

This could also be extended to each field for greater granularity.

type entity { player({readonly} id:int, stats:stats, inventory:list(item)) | enemy({readonly} id:int) }