Avoid invoking copy constructor when safe printing (#184)

Avoid invoking copy constructor when safe printing (#184)

When CVC4 gets interrupted, we use async-signal safe printing functions
to print statistics. Unfortunately, the code for that was invoking copy
constructors, which is problematic due to memory allocation; for example
with statistics such as ReferenceStat<std::string>. This commit adds a
getDataRef() method for statistics that returns a const reference to the
object being printed such that the copy constructor is not called. Note:
modifying getData() was unfortunately not an option because in the case
of TimerStat, we can't return a reference to an object on the stack. We
could remove the const modifier on getData() and use d_data to store the
information but then we would have to remove it on
safeFlushInformation() and potentially other methods as well, which
seems like a worse solution.