immutable.{HashMap, ChampHashMap} should use a serialization proxy

[retronym]

See also #523, which seeks to define a general strategy for serialization

HashMaps need to customize deserialization because element hash codes could be different on the deserizalizing JVM. immutable.HashMap has this in 2.13.x, but it hasn't been ported into this repo in either of the immutable hashmap implementations.

[lrytz]

There's an old (fixed) ticket for this: scala/bug#1600. I disabled the test case for now. Once this is fixed, test/files/jvm/t1600.scala needs to be re-enabled (marked with TODO-newColl).