8342588: Shenandoah: Pull shared closures together in one header

[shipilev]

Currently we have shenandoahClosures.hpp and shenandoahOopClosures.hpp, each of which have some closure inside. There is no reason to have this separation, and we should really just merge everything in one header. I also did a bit of mechanical renames. There should be no semantic change with this patch.

Additional testing:

• Linux x86_64 server fastdebug, hotspot_gc_shenandoah

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Issue

• JDK-8342588: Shenandoah: Pull shared closures together in one header (Enhancement - P4)

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[mlbridge]

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• 01: Full - Incremental (c9af382c)
• 00: Full (cbc94ff4)