Restore structural equality/hash code of the Type hierarchy
#261
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This commit adds an internal `Interned` interface to be implemented by Jandex classes that require non-standard equality and hash code for interning. This is the case for the `Type` hierarchy, as well as for classes that embed some `Type` (that is, the `Method/Field/RecordComponentInternal` classes). The reason why the `Type` hierarchy needs a different equality and hash code for intering is the existence of type variable references. To maintain structural equality/hash code for external users (that is, the common `equals` and `hashCode` methods), type variable references equality/hash code must only consider the type variable name (and annotations). This is not suitable for the interning purpose, because two type variable references may have the same name and annotations, yet point to different type variables (e.g. because those type variables have different annotations). We could possibly implement a "deep" structural equality and hash code for type variable references that would take into account the type variable the reference points to, but that still wouldn't be enough. When interning, types must be interned "from the inside", which means that when a type variable reference is being interned, the "target" type variable is not interned yet (possibly even not yet set) and can change later. This is one of the reasons why type variable references are mutable, and that in turn requires that their equality and hash code for interning purposes are based on identity. Note that type variable references are only mutated during indexing. After an `Index` is complete, type variable references must be considered "frozen".
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Fixes #258