misc cleanup of ML bindings (for multihreading);

added int type constraint to accomodate hacked SML/NJ (backported change in generated metis.ML);

preparations for proper type int;

tuned tracing;

replaced Theory.self_ref by Theory.check_thy, which now produces a checked ref;
thread-safeness: when creating certified items, perform Theory.check_thy *last*;

certify: no check_thy here;

improved check_thy: produce a checked theory_ref (thread-safe version);
removed obsolete self_ref (cf. check_thy);
thread-safeness: when creating certified items, perform Theory.check_thy *last*;
eq_thy: no check here;
marked some critical sections;

moved Admin/proper_int.ML to Pure/ML-Systems/proper_int.ML;

added dependency on Tools/Metis/metis.ML;

updated;