Uniform approach to serialization across collections

[retronym]

Before shipping 2.13.0, we should come up with a uniform approach to serialization of collections.

To date, our approach has been -

• make a collection Serializable
• later, remember to add @SerialVersionUID as needed
• later, switch to custom serialization (writeObject or writeReplace) to avoid directly serializing the internal structure, allowing us to change internals.

We also need to use custom serialization to deal with objects whose hash code changes on deserialization. We do this in mutable.HashMap. It appears to be missing from mutable.{HashMap, ChampHashMap} at the moment.

IMO, our default approach should be serializing a tag with the collection type, collection type specific config data (e.g. loadFactor), and then the elements. Deserialization would build a new collection with these elements. Details like class Map1, or the precise field layout of HashCollisionMapNode could be changed while remaining serialization compatible.

However, there is a gotcha with custom serialization: scala/bug#9237

In some classloader setups, we end up trying to deserialize the elements in the wrong classloader.

[lrytz]

cross-link scala/bug#6654 and scala/bug#7005 and scala/bug#8576