replace uses of U64_* and I64_* macros with their C99 stdint.h or inttypes.h equivalents

There are many macros in compat.h that try to portably handle 64-bit integer values in printf() and scanf() calls. Now that we seem to require C99, we should use the equivalent macros from stdint.h or inttypes.h instead. For example, U64_FORMAT is equivalent to PRIu64 in inttypes.h.