Avoid boxing in PrimitiveDictionaryBuilder

[tustvold]

Which issue does this PR close?

Closes #2215

Rationale for this change

What changes are included in this PR?

Are there any user-facing changes?

[codecov-commenter]

Codecov Report

Merging #2216 (3f80985) into master (8139f7b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2216   +/-   ##
=======================================
  Coverage   82.57%   82.57%           
=======================================
  Files         239      239           
  Lines       62269    62274    +5     
=======================================
+ Hits        51416    51421    +5     
  Misses      10853    10853           

Impacted Files	Coverage Δ
.../src/array/builder/primitive_dictionary_builder.rs	80.30% <100.00%> (+1.61%)	⬆️
...rquet/src/arrow/record_reader/definition_levels.rs	88.60% <0.00%> (-0.43%)	⬇️
parquet/src/encodings/encoding.rs	93.43% <0.00%> (-0.20%)	⬇️
parquet_derive/src/parquet_field.rs	66.21% <0.00%> (+0.45%)	⬆️

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[ursabot]

Benchmark runs are scheduled for baseline = acd8042 and contender = 985760f. 985760f is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-rs-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java