RFR: 8233708: VectorSet cleanup

[Claes Redestad]

Hi,

VectorSet needs a cleanup:

- Extravagant use of operator overloading (<<= adds items to the set,
 >>= removes them :eyeroll:)
- Plenty of unused methods
- Since VectorSet is the only implementation in the code base, the
abstract base class Set is unnecessary
- Various method names in conflict with HotSpot code "standard"

Webrev: http://cr.openjdk.java.net/~redestad/8233708/open.00/
Bug:    https://bugs.openjdk.java.net/browse/JDK-8233708

Testing: tier1-3, built and sanity tested with Shenandoah due changes
in some Shenandoah C2 support

Thanks!

/Claes