improved indentation

New function read_cterms is a combination of read_def_cterm and read_cterm. It reads and simultaneously type-checks a list of strings. cterm_of no longer catches exception TYPE; instead it must be caught later on and a message printed using Sign.exn_type_msg. More informative messages can be printed this way.

exports exn_type_msg for error messages. Calls new infer_types. Improved comments.