* compress sstable with zstd
* add some details to sstable readme
* compress only block which benefit from it
* multiple changes to sstable
make compression optional
use OwnedBytes instead of impl Read in sstable, required for next point
use zstd bulk api, which is much faster on small records
* cleanup and use bulk api for compression
* use dedicated byte for compression
* switch block len and compression flag
* change default zstd level in sstable
* Refactoring to prepare for the addition of dynamic fast field
- Exposing insert_key / insert_value
- Renamed SSTable::{Reader/Writer}-> SSTable::{ValueReader/ValueWriter}
- Added a generic Dictionary object in the sstable crate
- Removing the TermDictionary wrapper from tantivy, relying directly on
an alias of the generic Dictionary object.
- dropped the use of byteorder in sstable.
- Stopped scanning / reading the entire dictionary when streaming a range.
* Added a benchmark for streaming sstable ranges.
* CR comments.
Rename deserialize_u64 -> deserialize_vint_u64
* Removed needless allocation, split serialize into serialize and clear.
