clarified type ML_Context.antiq: context parser maintains compilation context, declaration is applied to final context; subtle change of semantics of ML_Context.add_antiq: need to avoid accidental update of context due to concrete syntax parser (cf. Scan.pass);

tuned signature;

discontinued unused antiquotation blocks;

missing ROOT (cf. beb4ee344c22);

more modular setup;

clarified position of Spec_Check for Isabelle/ML -- it is unrelated to Isabelle/HOL; just one src/Tools/ROOT;

removed unused ML antiquotations @{let}, @{note};