Reduce index size
- Compress ids, weights, values in posting block using zlib 715eb93e2663 .
- Encode term keys using a short code for the field name, saving the fieldname-code map with the term file.
- Encode term info using bytes for tf and df and int for offset when possible. When tf == df == 1 just store the offset.
- Encode stored fields as a list instead of a dict, storing the fieldname-pos map with the file.