Forward and backward binary compatibility between 2.10.0 and 2.10.1-RC1

Start running mima both ways, filtering out failures using @gkossakowski's [contribution to mima](https://github.com/gkossakowski/migration-manager/tree/13-cli-filters), which should be in 0.1.5-SNAPSHOT soon, to make sure bc.run doesn't fail (even where it should).

Each commit in this PR comments out the filters it makes irrelevant, until we only need to filter out permitted binary incompatibilities.

We only allow binary incompatibilities in scala.reflect.internal.


Reverts backports of:
  - SI-6521, overrides Range#head to be faster
  - SI-4664 Make scala.util.Random Serializable
  - SI-6548 reflection now correctly enters jinners

Also reverts/refactors:
  - cosmetic renamings in runtime reflection
  - duplicate tailImpl as a private method
  - bring back SerializeStart from fa3b804
  - can't add new class BatchingExecutor
  - refactor AdaptedForkJoinTask, uncaughtExceptionHandler
  - SI-6989 privateWithin is now populated in reflect (refactored, fix preserved)

review by/ping @retronym, @xeno-by, @gkossakowski, @paulp, @jamesiry, @phaller, @viktorklang and the rest of the world