Return t if OBJ is a nested alist.
Nested alist is a list of the form (ENTRY . BRANCHES), where ENTRY is
any Lisp object, and BRANCHES is a list of cons cells of the form
(KEY-ELEMENT . NESTED-ALIST).
You can use a nested alist to store any Lisp object (ENTRY) for a key
sequence KEYSEQ, where KEYSEQ is a sequence of KEY-ELEMENT. KEYSEQ
can be a string, a vector, or a list.