For the moment, this only allows for postings codec.
Also, on the write side, it does not include positions yet.
Implementation details:
On the write side, we use static typing.
A lot of types are now generics over the codec, but with a default codec type
that makes it so, we should not break client projects too much.
On the read side, we rely on a ObjectSafeCodec contraption to avoid
the proliferation of generics.
That object's point is to make sure we can build TermScorer with a concrete
codec specific type before reboxing it. (same thing for PhraseScorer).
* add method to fetch block of first vals in columnar
add method to fetch block of first vals in columnar (this is way faster
than single calls for full columns)
add benchmark
fix import warnings
```
test bench_get_block_first_on_full_column ... bench: 56 ns/iter (+/- 26)
test bench_get_block_first_on_full_column_single_calls ... bench: 311 ns/iter (+/- 6)
test bench_get_block_first_on_multi_column ... bench: 378 ns/iter (+/- 15)
test bench_get_block_first_on_multi_column_single_calls ... bench: 546 ns/iter (+/- 13)
test bench_get_block_first_on_optional_column ... bench: 291 ns/iter (+/- 6)
test bench_get_block_first_on_optional_column_single_calls ... bench: 362 ns/iter (+/- 8)
```
* use remainder
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.
