C++ header assumes 64-bit pointers

[lhoward]

Should likely use uintptr_t rather than uint64_t.

[zhuowei]

I went with uint64_t directly because:

• I only test this on x86-64, so even if I switched to uintptr_t/usize I'd likely break something else on 32-bit and not notice
• uint64_t is enough to hold a 32-bit pointer anyways at a slight cost in performance