volt: Magic tokens into expressions.

__FILE__ and __LINE__ are now expressions.
__FUNCTION__ and __PRETTY_FUNCTION__ have been added,
but both only return the unmangled simple name of the
current function at the moment.
semantic: Handle __FUNCTION__ correctly.
visitor: Allow PrettyPrinter to take a delegate sink.
semantic: Implement __PRETTY_FUNCTION__.
semantic: Handle __FILE__ and __LINE__ as Exps, too.