Add a bit-map encoding of Object-reference field offsets to ClassObject.

Add a bit-map encoding of Object-reference field offsets to ClassObject.

Class.c populates a new field with a bit for each of the first 32 instance
fields, to show if it is a reference or not, and an escape value if there is a
reference field beyond 32.

The GC uses the encoded bitmap -- if available -- to extract Object refererences
from DataObjects.