minor performance tuning, notably for Bytes.add (e.g. YXML output);

revert 90f6e541e926, which has become pointless thanks to df85df6315af;

clarified signature and modules;

clarified files;

drop pointless print_mode operations Output.output / Output.escape;

clarified signature: prefer explicit type Bytes.T;

clarified signature, roughly following Isabelle/Scala;

clarified print mode "latex": no longer impact Output/Markup/Pretty operations; renamed Output.output to Output.output_; removed Output.escape;

tuned comments, following Isabelle/ML;

tuned module structure;

tuned;

clarified unbreakable latex output: Pretty.unformatted and (Pretty.string_of o Pretty.unbreakable) should coincide, but are produced by quite different means;

clarified signature;

tuned signature;

tuned;

minor performance tuning, following Isabelle/Scala;

tuned;

tuned signature;

more scalable;

tuned;

eliminate print mode "code_presentation" thanks to value-oriented Pretty.T operations;

NEWS: value-oriented Pretty.T;

proper formal sections;

tuned signature: more options;

clarified Pretty.markup_block: use value-oriented YXML.output_markup, with final re-interpretation via print_mode in output_tree; clarified Pretty.symbolic: always use YXML.output_markup;

minor performance tuning;

unused;

prefer static YXML.output_markup_only (without print_mode): Output.status is only relevant for PIDE (with print_mode "isabelle_process");

clarified signature: more explicit type output_ops: default via print_mode;

discontinued unused global variable (see also following bf465f335e85);

tuned signature;

clarified modules (see also ea7c2ee8a47a);