error in bitbackerx8

[trinity-1686a]

There seems to be a copy-past error here, the index should be different from one another https://github.com/tantivy-search/bitpacking/blob/8cc0a1700e690de97b0bb4e0fb205024c0deaf3c/src/bitpacker8x.rs#L35-L36

[trinity-1686a]

I've re-implemented it on the playground with some test vectors, the correct indices are 0 and 4.

I have to point out that variable names are misleading : _mm256_srli_si256 operate on 2 128b lanes, so that a,b,c,d and e,f,g,h can't get mixed, a__b__ca_db_ce_df_ge_hf should hence be a__b__ca_db_e__f__ge_hf (same goes for ___a__b__ca_db_ce_df_ge).