cast dictionaries to i64 key type to avoid generic explosion

[davidhewitt]

This PR changes dictionary handling to eagerly upcast the keys to i64 size.

This allows us to avoid changing any logic but strip a layer of generics away, namely the possible dictionary keys. This results in a ~20x reduction in code size as measured by cargo llvm-lines:

before

Lines                  Copies                Function name
  -----                  ------                -------------
  6187111                209087                (TOTAL)
   657720 (10.6%, 10.6%)  12600 (6.0%,  6.0%)  datafusion_functions_json::common::zip_apply::inner::{{closure}}
   598662 (9.7%, 20.3%)    3789 (1.8%,  7.8%)  <arrow_array::array::primitive_array::PrimitiveArray<T> as core::iter::traits::collect::FromIterator<Ptr>>::from_iter
   552270 (8.9%, 29.2%)     280 (0.1%,  8.0%)  datafusion_functions_json::common::zip_apply

after

Lines                 Copies              Function name
  -----                 ------              -------------
  305725                9773                (TOTAL)
   34870 (11.4%, 11.4%)   60 (0.6%,  0.6%)  datafusion_functions_json::common::zip_apply
   25200 (8.2%, 19.6%)   480 (4.9%,  5.5%)  datafusion_functions_json::common::zip_apply::inner::{{closure}}
   24174 (7.9%, 27.6%)   153 (1.6%,  7.1%)  <arrow_array::array::primitive_array::PrimitiveArray<T> as core::iter::traits::collect::FromIterator<Ptr>>::from_iter

[samuelcolvin]

LGTM.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 69.15888% with 33 lines in your changes missing coverage. Please review.

Project coverage is 83.28%. Comparing base (82521b6) to head (e025824).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/common.rs	69.15%	24 Missing and 9 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #66      +/-   ##
==========================================
- Coverage   83.82%   83.28%   -0.55%     
==========================================
  Files          15       15              
  Lines        1045     1089      +44     
  Branches     1045     1089      +44     
==========================================
+ Hits          876      907      +31     
- Misses        120      125       +5     
- Partials       49       57       +8     

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