[Dexter] Add !address node
Adds a node type for Dexter that allows checking abstract labels instead of
concrete addresses. Each address node has a label and optional offset, and
the first time during evaluation that a given address label is matched
against a valid pointer value, the address label will be assigned a value
that matches the seen address (adjusting for any offset). From that point,
the resolved address value will be used for the remainder of the test
evaluation.
