RFC: Use a more general but still object-safe signature for Query::query_terms.

[adamreichold]

BTreeMap is a relatively specific data structure and the current signature enforces allocating a new collection and cloning the terms even if the consumer only needs to inspect a subset of the terms.

This PR is a breaking change which aims to avoid this by using a closure to visit each of the terms contained in the query. The closure is passed as a trait object to keep the Query trait it self object-safe.

The approach has the downside of pushing deduplication of identical terms into the visitor. But it also gives consumers the option of e.g. counting multiplicities.

[codecov-commenter]

Codecov Report

Merging #1468 (c39f03b) into main (8b3a6f6) will decrease coverage by 0.00%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##             main    #1468      +/-   ##
==========================================
- Coverage   94.09%   94.09%   -0.01%     
==========================================
  Files         238      238              
  Lines       44552    44538      -14     
==========================================
- Hits        41923    41908      -15     
- Misses       2629     2630       +1     

Impacted Files	Coverage Δ
src/query/boost_query.rs	69.76% <0.00%> (ø)
src/query/disjunction_max_query.rs	0.00% <0.00%> (ø)
src/query/query.rs	85.71% <66.66%> (ø)
src/query/boolean_query/boolean_query.rs	92.76% <100.00%> (ø)
src/query/mod.rs	100.00% <100.00%> (ø)
src/query/phrase_query/phrase_query.rs	91.54% <100.00%> (ø)
src/query/term_query/term_query.rs	93.02% <100.00%> (ø)
src/snippet/mod.rs	90.02% <100.00%> (+0.30%)	⬆️
src/fastfield/reader.rs	87.73% <0.00%> (-0.62%)	⬇️
src/store/index/mod.rs	97.83% <0.00%> (-0.55%)	⬇️

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