1) Body descriptors moved to their own header files.
2) Missing body descriptors added.
3) Template versions of HeapObject::Iterate*() methods added.
4) Body descriptors support new kind of queries: IsValidSlot(offset) which can be used for invalid slots filtering.

This is a first step towards virtual and static visitors unification and support in-object properties in built-in (sub-)classes.

Review URL: https://codereview.chromium.org/1440243002

Cr-Commit-Position: refs/heads/master@{#31980}