debug_printf requires unsafe

[charles-r-earp]

The debug_printf and debug_printfln macros generate an invocation of the asm macro, which requires an unsafe block. However, the macro appears to validate it's input, so it could potentially add the unsafe block to it's output. If there are still outstanding issues, perhaps a safety section could be added to the docs to reduce confusion? This is a really useful tool, and it seems strange to require unsafe just to debug stuff.