Consider using covariance in generated interfaces' collections

We currently generate interfaces which result in:

List<Foo> getFoo();

i.e. we are using invariant type for the returned List. This is kind of okay and is readable by novices, but has a downside - there are no compile-time guards against attempted mutation of the collection - which usually will end up blowing up at runtime.

We should consider returning covariants, such that the above becomes:

List<? extends Foo> getFoo();

This has the benefit of allowing reads from the List, but writes are stopped cold by the compiler - you just cannot put anything into the list (unless you battle the type system and perform a raw/unchecked cast).