(ewoc--adjust): New func.

(ewoc--adjust): New func.
(ewoc--insert-new-node): Don't insert trailing newline.
Instead, adjust succesor nodes' start markers.
(ewoc--refresh-node): Delete all text from current node's start
marker to the next one's; adjust successor nodes' start markers.
(ewoc--create): Doc fixes.
(ewoc--refresh): Don't insert newline.
(ewoc--set-hf): Use `ewoc--set-buffer-bind-dll-let*'.