want ::v8whatis

[davepacheco]

See #107 for context. This ticket covers only the lower-level V8-level dcmd called ::v8whatis. This takes an address and attempts to find the V8 heap object containing that address (by looking at the memory below that address until it finds a Map, then looking to see if that address represents a valid object that would contain the target pointer).

[davepacheco]

I opted not to tie this into ::whatis yet (as mentioned in #107) for two reasons: I still want to see how useful this turns out to be, and I think it might be more useful to reserve ::whatis support for a JS-level analog.

Review:
https://cr.joyent.us/#/c/3520/

PS2 is make check clean and passes all tests in "runtest_node" (all Node versions tested), including the new tests for the new dcmd.

[davepacheco]

I've updated the change for a few more nits. I re-ran runtest_node (retesting all Node versions we usually test) and make check on PS3. The only issue was that I ran into #38 on the 32-bit 0.12.17 test. I re-ran it with no issues.