RFR (S) 8241740: Shenandoah: remove ShenandoahHeapRegion::_heap

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8241740

It is not seem to worth it to drag the _heap field in every region. Almost all the places where it
is used is not performance-critical, and ShenandoahHeap::heap() is actually as fast, especially
after JDK-8241743. Ditching this field saves 8 bytes per region.

I had to move seqnum_last_alloc_mutator to avoid including shenandoahHeap.inline.hpp in a generic
header.

Webrev:
  https://cr.openjdk.java.net/~shade/8241740/webrev.02/

Testing: hotspot_gc_shenandoah, eyeballing shenandoahHeapRegion.o objdumps

-- 
Thanks,
-Aleksey