make(1): make the API of the List partially public

Accessing the fields List.first, List.last, ListNode.prev, ListNode.next
and ListNode.datum in read-only mode should be more efficient than a
whole function call.

All modifications to the lists or their nodes must still happen via
function calls.

This change reduces the code size, makes the code faster to execute and
allows Lst_ForEach to be written inline without the visual overhead of
function calls.

(rillig)