added theory_data.ML;

Type-safe interface for theory data.

* improved the theory data mechanism to support real encapsulation; main change of the internal interfaces: data kind name (string) replaced by private Object.kind, acting as authorization key;

accomodate tuned version of theory data;

added print_theorems: theory -> unit; added print_theory: theory -> unit; added transaction mechanism as last resort to accomodate non-atomic transformers (please avoid such things); tuned setup;

Object.T;

improved data: secure version using Object.T and Object.kind;