prefix operator for printf debugging

[remexre]

It'd be convenient for debugging to have a prefix operator (or some other syntax) that expands like %x => unsafeTracePrint(x, __FILE__ ++ __LINE__ ++ show(x)) (not literally that for thunk-sharing, formatting, etc. reasons. (see Rust's dbg! for prior art)

[krame505]

Seems reasonable, not sure what the best syntax would be. Would prefer to not just use a symbol, but I would also like to minimize the number of things we add that look like functions but aren't.