radix_array: Bound span() and base_span() at N
radix_array: Bound span() and base_span() at N

Previously, index()+span() and base()+base_span() could return indexes
beyond end().  This was unexpected and error prone.  Fix this by
capping the returned spans appropriately.

bowlofstew/sv6,aclements/sv6,bowlofstew/sv6,aclements/sv6,bowlofstew/sv6,bowlofstew/sv6,bowlofstew/sv6,aclements/sv6,aclements/sv6,aclements/sv6