Make boxed math warning suppressible

So I decided to take a shot at writing a patch for this. This is my first Clojure core patch, so I've probably messed up some formatting, but the implementation was pretty simple and the tests pass.

I introduced a variable, clojure.core/silence-boxed which defaults false and, when true, silences boxed math warnings. If the reverse is preferred (warn-boxed or similar) I can do that too.

Hi Michael, we have other plans for how this should be implemented, so will likely not use your patch. In the future, it's always good to check if the ticket is already assigned to someone before working on it.

Added clj-1535-3.patch, which is exactly the same diff as clj-1535-2.patch, but just squashes into a single commit.