src: fix object inspection for V8 6.4

[mmarchini]

V8 6.4 "replaces the in-object properties count byte in the map
with the byte that stores the start offset of in-object properties".
Object inspection on llnode relies on in-object properties being count
bytes, so these are minimal changes to make object inspection work again
with V8 6.4 while keeping compatibility with previous versions.

Ref: https://chromium-review.googlesource.com/c/v8/v8/+/776720
Fixes: #158

[mmarchini]

I thought it only applies when the map's instance type >= FIRST_JS_OBJECT_TYPE, i.e., when it's a map describing a JSObject.

There are 5 calls to InObjectProperties in llnode, 4 of them are in JSObject methods, so those calls should be safe. The only one I'm not 100% sure is correct is the call in Map::Inspect. We might need to do some special handling there, but I'm not sure. I'll leave a FIXME comment to investigate this in a follow-up PR, WDYT?

[mmarchini]

Improved legibility on InObjectProperty and handled the case where Map don't describe a JSObject. @bnoordhuis PTAL.

Would be nice to have a test for this case. I'll see if I can come up with one later, but suggestions are appreciated 😄