Blocked by scala/scala3#7174

There are many changes here that prepare for Dotty that also work fine for Scala 2.x series. Merge these changes to avoid rebasing for future Dotty support. The 52 test errors are in the following category:

1. Serialization
2. Null cache problem

There are also about 20 non-local return warnings but all other compile time warnings and errors have been cleared for both the library and test code.