Retrieves a value of a slot from a hierarchy of objects. A nil on
the path is ignored.

ex:
(slot-value-by-path employee '(address street)) => “17 Sunvalley St.”
(slot-value-by-path employee '(address)) => #<ADDRESS {XXX}>
(slot-value-by-path employee 'address) => #<ADDRESS {XXX}>
(slot-value-by-path address '(street)) => “17 Sunvalley St.”
(slot-value-by-path address '(nil street)) => “17 Sunvalley St.”

obj - a CLOS object
path - a list of slot names