Rewrite nested jar support code and remove Java 8 support

[philwebb]

Our current nested jar support is pretty complicated due to support for earlier versions of Java. There are also a lot of bugs that we're unable to fix with the current implementation. Rewriting our implementation would help us a great deal going forward.

[philwebb]

Reopening due to Windows build issues:

java.io.IOException: Failed to delete temp directory C:\Windows\TEMP\junit17935825065843241783. The following paths could not be deleted (see suppressed exceptions for details): , test.jar