Fix Int64 <-> Int errors on 32-bit architectures

- Size unpacking (unsigned, big-endian integers) were broken on 32-bit
  systems due to fixed Int64 specifiers.
- Converting size values to Uint64 seems to solve this issue.
- Even if Erlang supports only 4-bytes wide size values, in some cases
  we need to track larger lengths.  Therefore Uint64 may seem a good
  choice for tracking sizes.