See extended description

1. Changes to fix bus error wrt TPL
2. Fixed incomplete handling of string objects in multiple places
(string literals should also be considered)
3. equal() handles strings now
4.  Added SYSTEM utility operator for executing system calls from within
pLisp
5. Unit test cases added (all special operators now covered)
6. Converted more occurrences of throw_generic_exception() to
throw_exception()