At the very least, the itoap crate being used there does divisions by and comparisons to powers of 10 as part of stringifying integers, which the C and ASM vers

At the very least, the itoap crate being used there does divisions by and comparisons to powers of 10 as part of stringifying integers, which the C and ASM versions don't seem to be doing. A crate with a function for stringifying arbitrary integers cannot take advantage of the fact that it's going to be used for stringifying consecutive integers, as an inlined solution can.