add constructors for Builtin Feature Structure lists that take value and maybe the next ptr

I think it would be frequently convenient to have a constructor for a FS list (e.g. NonEmptyFloatList), that took one or 2 additional args. The first additional argument would be the value to set, the 2nd optional argument would be the value for the "tail" pointer - if omitted, it would set it to the (common, shared) "empty" value for this kind of list.

Also, rework getEmptyList() so it works better with existing instances of the 4 kinds of lists.