Tantivy

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- Bugfix in the filesum check. (evanxg852000) 1127
- Bugfix in positions when the index is sorted by a field. (appaquet) 1125

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- Major bugfix. Deleting documents was broken when the index was sorted by a field. (appaquet, fulmicoton) 1101

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- Major bugfix. DocStore still panics when a deleted doc is at the beginning of a block. (appaquet) 1088

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- Major bugfix. DocStore panics when first block is deleted. (appaquet) 1077

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- API Changes. Using Range instead of (start, end) in the API and internals (`FileSlice`, `OwnedBytes`, `Snippets`, ...)
This change is breaking but migration is trivial.
- Added an Histogram collector. (fulmicoton) 994
- Added support for Option<TCollector>. (fulmicoton)
- DocAddress is now a struct (scampi) 987
- Bugfix consistent tie break handling in facet's topk (hardikpnsp) 357
- Date field support for range queries (rihardsk) 516
- Added lz4-flex as the default compression scheme in tantivy (PSeitz) 1009
- Renamed a lot of symbols to avoid all uppercasing on acronyms, as per new clippy recommendation. For instance, RAMDirectory -> RamDirectory. (fulmicoton)
- Simplified positions index format (fulmicoton) 1022
- Moved bitpacking to bitpacker subcrate and add BlockedBitpacker, which bitpacks blocks of 128 elements (PSeitz) 1030
- Added support for more-like-this query in tantivy (evanxg852000) 1011
- Added support for sorting an index, e.g presorting documents in an index by a timestamp field. This can heavily improve performance for certain scenarios, by utilizing the sorted data (Top-n optimizations)(PSeitz). 1026
- Add iterator over documents in doc store (PSeitz). 1044
- Fix log merge policy (PSeitz). 1043
- Add detection to avoid small doc store blocks on merge (PSeitz). 1054
- Make doc store compression dynamic (PSeitz). 1060
- Switch to json for footer version handling (PSeitz). 1060
- Updated TermMerger implementation to rely on the union feature of the FST (scampi) 469
- Add boolean marking whether position is required in the query_terms API call (fulmicoton). 1070

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- Remove dependency to atomicwrites 833 .Implemented by fulmicoton upon suggestion and research from asafigan).
- Migrated tantivy error from the now deprecated `failure` crate to `thiserror` 760. (hirevo)
- API Change. Accessing the typed value off a `Schema::Value` now returns an Option instead of panicking if the type does not match.
- Large API Change in the Directory API. Tantivy used to assume that all files could be somehow memory mapped. After this change, Directory return a `FileSlice` that can be reduced and eventually read into an `OwnedBytes` object. Long and blocking io operation are still required by they do not span over the entire file.
- Added support for Brotli compression in the DocStore. (ppodolsky)
- Added helper for building intersections and unions in BooleanQuery (guilload)
- Bugfix in `Query::explain`
- Removed dependency on `notify` 924. Replaced with `FileWatcher` struct that polls meta file every 500ms in background thread. (halvorboe guilload)
- Added `FilterCollector`, which wraps another collector and filters docs using a predicate over a fast field (barrotsteindev)
- Simplified the encoding of the skip reader struct. BlockWAND max tf is now encoded over a single byte. (fulmicoton)
- `FilterCollector` now supports all Fast Field value types (barrotsteindev)
- FastField are not all loaded when opening the segment reader. (fulmicoton)
- Added an API to merge segments, see `tantivy::merge_segments` 1005. (evanxg852000)

This version breaks compatibility and requires users to reindex everything.