Issue/822 TopDocs sorted by i64, and date fastfield (in addition to u64) (#890)

* Unsatisfactory implementation.

The fastfield are hit. But for performance, we want the comparison to happen on u64,
and the conversion to the FastType to be done only on the selected TopK
elements.

For i64, the current approach might be ok.
For DateTime, it is most likely catastrophic.

Closes #822

* Decoupled SegmentCollector Fruit from Collector Fruit.

Deferred conversion from u64 to the proper FastField type to after the overall collection.
(tantivy guarantees that u64 encoding is consistent with the original
ordering of the fastfield)

Closes #882

src/error.rs

@@ -117,6 +117,12 @@ impl<Guard> From<PoisonError<Guard>> for TantivyError {
     }
 }
 
+impl From<chrono::ParseError> for TantivyError {
+    fn from(err: chrono::ParseError) -> TantivyError {
+        TantivyError::InvalidArgument(err.to_string())
+    }
+}
+
 impl From<schema::DocParsingError> for TantivyError {
     fn from(error: schema::DocParsingError) -> TantivyError {
         TantivyError::InvalidArgument(format!("Failed to parse document {:?}", error))