moved test-case that causes stack-overflow...

...to special directory
(and yes, the stack-overflow is caused by the input rather than a bug on
our side)
Added new standard AST-node visitor class

...returns constant default value for everything you don't override
Fixed pretty printer...

...and tests, meaning:
PrettyPrinting an expression should NOT take care of wrapping it in
parentheses at ANY TIME!
That is definitely the responsibility of the caller rather than the
callee, as it cannot possibly decide whether parentheses are required or
not (without using some magical global variable that is crazy to
maintain).