Add submodules which define type aliases for the `byteorder::*` types
with a specific byte order type argument. This allows us to go from the
somewhat clunky:

  use byteorder::NetworkEndian;
  use zerocopy::byteorder::U16;

  #[derive(FromBytes, AsBytes, Unaligned)]
  #[repr(C)]
  struct UdpHeader {
      src_port: U16<NetworkEndian>,
      dst_port: U16<NetworkEndian>,
      length: U16<NetworkEndian>,
      checksum: U16<NetworkEndian>,
  }

...to the less clunky:

  use zerocopy::byteorder::network_endian::U16;

  #[derive(FromBytes, AsBytes, Unaligned)]
  #[repr(C)]
  struct UdpHeader {
      src_port: U16,
      dst_port: U16,
      length: U16,
      checksum: U16,
  }

Closes #100