RFR: Move HdrSeq and BinaryMagnitudeSeq into Shenandoah utilities

[Roman Kennke]

Very good, thank you!

Roman

> http://cr.openjdk.java.net/~shade/shenandoah/move-seqs/webrev.01/
> 
> This alleviates upstream exposure by hiding the implementation down in Shenandoah-specific files.
> Had to do the separate file to avoid circular inclusion problems.
> 
> Testing: tier1_gc_shenandoah {fastdebug|release}
> 
> Thanks,
> -Aleksey
>