moved global pretty/string_of functions from Sign to Syntax; reordered signature;
moved global pretty/string_of functions from Sign to Syntax;
reordered signature;

MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle