avoid duplicate markup, notably from "CONST c";

clarified pretty_entity for syntax consts without mixfix annotation (see also 43c4817375bf and d622145603ee);

tuned;

clarified signature (see also 2157039256d3);

more syntax bundles, e.g. to explore terms without notation;

commands 'syntax_types' and 'syntax_consts' now work in a local theory context;

tuned names;

tuned signature: avoid shadowing;

syntax translations now work in a local theory context;

clarified signature;

clarified signature and modules;

minor performance tuning: avoid excessive Library.insert and Symbol.explode operations;

tuned whitespace;

tuned proofs;

tuned proofs;