more efficient Markup_Tree, based on branches sorted by quasi-order;
renamed markup_node.scala to markup_tree.scala and classes/objects accordingly;
Position.Range: produce actual Text.Range;
Symbol.Index.decode: convert 1-based Isabelle offsets here;
added static Command.range;
simplified Command.markup;
Document_Model.token_marker: flatten markup at most once;
tuned;
no_document use_thy "Setup";
use_thy "Introduction";
use_thy "Foundations";
use_thy "Refinement";
use_thy "Inductive_Predicate";
use_thy "Adaptation";
use_thy "Further";