[wadler] Prettier handling of trivia

Split the rules for pre- and post- trivia, understand when we want to
do either, handle multi-line-break (in an unsatisfying way, I guess)
but otherwise lay the groundwork for thinking about it better.

Also now we don't generate lazy "Text" nodes because I thought I might
want to actually look at the newlines in the source but I don't yet.
I *can* now, though. (I can also detect EOF so there's that.)

harness.py

@@ -371,7 +371,7 @@ class Harness:
 
             printer = self.load_printer()
             if self.tree is not None:
-                self.document = printer.convert_tree_to_document(self.tree)
+                self.document = printer.convert_tree_to_document(self.tree, self.source)
             else:
                 self.document = None
 
@@ -541,12 +541,6 @@ class Harness:
                 append(f"indent {doc.amount}")
                 self.format_document(lines, doc.doc, indent + 1)
 
-            case wadler.Text(start, end):
-                if self.source is not None:
-                    append(f"< {repr(self.source[start:end])}")
-                else:
-                    append(f"< ??? {start}:{end}")
-
             case wadler.Literal(text):
                 append(f"literal {repr(text)}")