[AutoDiff] NFC: Reimplement JVPCloner using pimpl pattern.

`JVPCloner.h` is now tiny: `JVPCloner` exposes only a `bool run()` entry point.

All of the implementation is moved to `JVPCloner::Implementation` in
`JVPCloner.cpp`. Methods can be defined directly in `JVPCloner.cpp` without
separate declarations.

Author: dan-zheng