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;
theory Isar
imports Base
begin
chapter {* Isar language elements *}
text {*
The primary Isar language consists of three main categories of
language elements:
\begin{enumerate}
\item Proof commands
\item Proof methods
\item Attributes
\end{enumerate}
*}
section {* Proof commands *}
text FIXME
section {* Proof methods *}
text FIXME
section {* Attributes *}
text FIXME
end