Do not assume that immutable collections contain immutable types.

Right now the `Immutable` rule simply assumes that an immutable collection (`Map`, `Set`, and `List`) are immutable.

But, of course there can be mutable types within the collection. Verify the type(s) used in the collection.

Start by locating the `Immutable` rule and its handling of `Map`, `Set`, and `List`. Trace how the types used inside each collection are checked, then add or update coverage so collections containing mutable types are not treated as immutable.

Written by the indexing model from the issue text.