* 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.
