Make ::findjsobjects output size

[ghost]

Currently ::findjsobjects will output number of objects, properties and constructor/properties.
It would be good to have the size.

Right now I have an approximation in some cases using ! wc -c but this doesn't seem very good.

[davepacheco]

I don't know how it's possible to get at the underlying object size, but this would be a nice feature. There's an instance_size field in the underlying V8 class that may be promising.