support XOR-based bit mixing in heavy_hash for header_version 2 and misc

[x100111010]

supports the upcoming proposed changes in spectre-project/rusty-spectre#41

  if header_version == 2 {
       (((sum1 & 0xF) ^ ((sum1 >> 4) & 0xF) ^ ((sum1 >> 8) & 0xF)) << 4) as u8
           | ((sum2 & 0xF) ^ ((sum2 >> 4) & 0xF) ^ ((sum2 >> 8) & 0xF)) as u8
   } else {
       ((sum1 >> 10) << 4) as u8 | (sum2 >> 10) as u8
   }

In version 1 we just took the upper bits of calculated sums (using sum >> 10).

In version 2 we now use XOR operations between different parts of the sum ((sum & 0xF) ^ ((sum >> 4) & 0xF) ^ ((sum >> 8) & 0xF)) which introduces much more randomness and variation in the output.