Split walkPath's four modes into walkers of their own, sharing how a path steps

hold.go

@@ -44,7 +44,7 @@ func readField(s *session, t *template, held *hold, sc renderScope, a arm) draw
 	if len(a.tail) > 0 {
 		sc.pins = &d.pins
 	}
-	r := renderLeaf(s, drawPath(t.head(a.key), a.tail, a.key, &d.pins, &pathDraws{s: s, held: d, a: &a}), sc)
+	r := renderLeaf(s, drawPath(t.head(a.key), a.tail, a.key, &renderWalk{s: s, pins: &d.pins, held: d, a: &a}), sc)
 	if d.value == nil {
 		d.value = map[string]draw{}
 	}