want tool for printing V8-level object details

[davepacheco]

When debugging cases where an object is missing properties or has incorrect properties, one often needs to rummage through internal V8 structures, and I usually wind up adding debug printouts to jsobj_properties(). It would be useful to first-class this with a dcmd that could print out:

• for all V8 heap objects: the address, the expected size in bytes of the object (including variable-sized pieces), and the ending address of the object
• for JSObjects in particular, all of the information used to find property names and values, including the property descriptors array, and for each property: the index into the descriptor array, the type, the name, the value, and so on