centralized handling of built-in types and constants for bitvectors

moved smt_word.ML into the directory of the Word library

centralized handling of built-in types and constants; also store types and constants which are rewritten during preprocessing; interfaces are identified by classes (supporting inheritance, at least on the level of built-in symbols); removed term_eq in favor of type replacements: term-level occurrences of type bool are replaced by type term_bool (only for the translation)