Silence compiler warnings for unused variables

I like to compile my libs with as many compiler warnings as possible. Sadly, including jsoncons.hpp pulls in lots of new ones. The unfortunate result is that I have to crank down the warnings on my own project.

As for unused args in particular, it helps with for example, variadic templates where some bad copy/paste and a left-out argument wouldn't cause a compile error.

If you're interested in pulling, I'd be happy to submit more cleanups. Using clang's -Weverything is great for finding subtle bugs, especially -Wsign-conversion and friends when dealing with serializing data.