[saffron] cleanup query

[martyall]

Summary

@marcbeunardeau88 did the initial index arithmetic to make the query interface, this PR just cleans up that work. I added additional tests to cover edge cases and attempted to make it more idiomatic (in an opinionated fashion).

The commits are atomic, individual diffs should be clear

Changes

• add an error type for malformed queries and force construction of well formed queries through smart constructor into_query_field. Added tests. 416ed93
• remove hard coded use of values like 31 (482964f, ffa98c7) and 2 ^ 16 (416ed93)
• Use the iterator to fulfill the query 3935f75
• Added test to check that the nil query doesn't throw an error af1b31b

[codecov]

Codecov Report

Attention: Patch coverage is 96.21212% with 5 lines in your changes missing coverage. Please review.

Please upload report for BASE (marc/query-saffron@e3253d3). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
saffron/src/utils.rs	96.21%	5 Missing ⚠️

Additional details and impacted files

@@                  Coverage Diff                  @@
##             marc/query-saffron    #2975   +/-   ##
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  Coverage                      ?   76.91%           
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  Files                         ?      261           
  Lines                         ?    61520           
  Branches                      ?        0           
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  Hits                          ?    47317           
  Misses                        ?    14203           
  Partials                      ?        0           

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