Complete implementation of comparison operators.

- works with NIL
- handles integers, strings, records, and arrays
- added a test case tests/gc0.tig that uses this
Report an error for undefined functions.

Before, the exception Not_found would be thrown
and no error information would be shown.
Cleanup.
Fix bug in evaluation of WHILE and FOR loops.

The loops were written as recursive functions which
were NOT tail-recursive, so loops could fail due to
stack overflow.