* Use a more general but still object-safe signature for Query::query_terms.
* Further constraint the generalized Query::query_terms signature to allow extracting references to terms.
- Change in the DocSet and Scorer API. (@fulmicoton).
A freshly created DocSet point directly to their first doc. A sentinel value called TERMINATED marks the end of a DocSet.
`.advance()` returns the new DocId. `Scorer::skip(target)` has been replaced by `Scorer::seek(target)` and returns the resulting DocId.
As a result, iterating through DocSet now looks as follows
```rust
let mut doc = docset.doc();
while doc != TERMINATED {
// ...
doc = docset.advance();
}
```
The change made it possible to greatly simplify a lot of the docset's code.
- Misc internal optimization and introduction of the `Scorer::for_each_pruning` function. (@fulmicoton)
* Alternative take on boosted queries
* Fixing unit test
* Added boosting to the query grammar.
* Made BoostQuery public.
* Added support for boosting field in QueryParser
Closes#547
