RFR: Some smallish ShHeapRegionSet changes

[Roman Kennke]

I extract this from the big partial-traversal patch.

1. Move constructor definition from .inline.hpp file to make compiler
happy (on aarch64). Else we'd have to mark it inline to avoid multiple
definition error.

2. Define assignment/copy operator. I need this in partial-traversal to
allow code like _root_regions_iterator = ...

Test: hotspot_gc_shenandoah

http://cr.openjdk.java.net/~rkennke/shhrs-changes/webrev.00/

Ok?

Thanks,
Roman