Declares the function exit_use to behave like use but fail if errors are detected. It can be used in all Makefiles except Pure, which will write the exception handler explicitly ("exit" will have been declared already).

Now the "use" call has an exception handler, for prompt failure if errors are detected.

Now calls exit_use instead of use, for prompt failure if errors are detected.

Removed an old bug which made some simultaneous instantiations fail if they were given in the "wrong" order. Rewrote sign/infer_types. Fixed some comments.

added exit 1

Removed superfluous type constraint

Changed treatment of during type inference internally generated type variables. 1. They are renamed to 'a, 'b, 'c etc away from a given set of used names. 2. They are either frozen (turned into TFrees) or left schematic (TVars) depending on a parameter. In goals they are frozen, for instantiations they are left schematic.