FIX: core: remove sprintf calls

This changes the behavior of ufunc_repr to no longer truncate its argument's name. I'm guessing the truncation was only there to make the function work with sprintf, and if I read http://bugs.python.org/issue7330 correctly, then truncation wouldn't work in 3.2 <= Python <= 3.4.
Yeah, looks like `signature` was a bug. BTW, I'm seldom responsible for a lot of code that `git blame` assigns to me, as I only show up because I made extensive style fixes throughout the code. The downside of style fixes is the loss of history.