use groupby instead of vec allocation

[PSeitz]

get_sorted_field_values is quite expensive and usually visible in traces. The allocations are unnecessary though.

Before (13 mulitivalue fast fields)
-    9,72%     1,21%  thrd-tantivy-in  tantivy               [.] tantivy::indexer::segment_writer::SegmentWriter::add_document                                                         
   - 8,51% tantivy::indexer::segment_writer::SegmentWriter::add_document                                                                                                               
      + 3,03% tantivy::store::store_compressor::BlockCompressor::compress_block_and_write                                                                                              
      + 2,42% tantivy::schema::document::Document::get_sorted_field_values                                                                                                             
      + 1,89% cfree                                                                                                                                                                    
        0,95% tantivy::fastfield::multivalued::writer::MultiValuedFastFieldWriter::add_document                                                                                        


After
-    8,10%     1,65%  thrd-tantivy-in  tantivy               [.] tantivy::indexer::segment_writer::SegmentWriter::add_document                                                        
   - 6,45% tantivy::indexer::segment_writer::SegmentWriter::add_document                                                                                                               
      + 3,52% tantivy::store::store_compressor::BlockCompressor::compress_block_and_write                                                                                              
        1,28% tantivy::fastfield::multivalued::writer::MultiValuedFastFieldWriter::add_document                                                                                        
        1,21% cfree                                                                                                                                                                    
   + 1,65% __clone                                                                                                                                                                     

[codecov-commenter]

Codecov Report

Merging #1582 (b5919ff) into main (44e0379) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1582      +/-   ##
==========================================
- Coverage   93.79%   93.78%   -0.02%     
==========================================
  Files         251      251              
  Lines       46329    46332       +3     
==========================================
- Hits        43456    43454       -2     
- Misses       2873     2878       +5     

Impacted Files	Coverage Δ
src/indexer/segment_writer.rs	96.39% <100.00%> (+0.01%)	⬆️
src/schema/document.rs	92.81% <0.00%> (-1.31%)	⬇️
src/fastfield/multivalued/mod.rs	97.23% <0.00%> (-1.19%)	⬇️
src/postings/segment_postings.rs	98.95% <0.00%> (-0.53%)	⬇️
.../termdict/sstable_termdict/sstable/block_reader.rs	82.14% <0.00%> (+1.78%)	⬆️

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