Fix definitions of ULONG_PTR
The Windows type `ULONG_PTR` is supposed to be equivalent to `usize`, but several parts of the codebase currently define it as `u64`. Evidently this hasn't broken anything yet but it might cause annoying 32-bit-specific breakage in future.
See https://msdn.microsoft.com/en-gb/library/windows/desktop/aa383751(v=vs.85).aspx
r? @alexcrichton
2527f41baf