RFR (S): ObjArrayFromToTask -> SCMTask

[Aleksey Shipilev]

Hi,

I would like to properly alias ObjArrayFromToTask as SCMTask:
 http://cr.openjdk.java.net/~shade/shenandoah/mark-scmtask/webrev.01/

This also introduces a few other renames. Plus a micro-optimization for do_task
that accepts SCMTask now, not the exploded fields. This matters since do_task is
large enough to be denied inlining: passing three arguments instead of one adds
up for many tasks to process. This patch is the per-requisite for the larger
follow-up patch with ObjArrayFromToTask performance improvements.

Testing: hotspot_gc_shenandoah, selected benchmarks

Thanks,
-Aleksey