Don't use PRIx64 formatting derives from inttypes.h

As the tinyformat-based formatting system (introduced in b77dfdc) is
type-safe, no special format characters are needed to specify sizes.

Tinyformat can support (ignore) the C99 prefixes such as "ll" but
chokes on MSVC's inttypes.h defines prefixes such as "I64X". So don't
include inttypes.h and define our own for compatibility.

(an alternative would be to sweep the entire codebase using sed -i to
get rid of the size specifiers but this has less diff impact)