Add WriteString method benchmarks for primitive objects

[adrg]

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[codecov]

Codecov Report

Merging #366 into v3 will decrease coverage by 5.98%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##               v3     #366      +/-   ##
==========================================
- Coverage   61.21%   55.23%   -5.99%     
==========================================
  Files         151      151              
  Lines       27251    27222      -29     
==========================================
- Hits        16682    15035    -1647     
- Misses      10107    10339     +232     
- Partials      462     1848    +1386

Impacted Files	Coverage Δ
pdf/creator/utils.go	55.55% <0%> (-22.23%)	⬇️
pdf/core/security/handlers.go	33.33% <0%> (-16.67%)	⬇️
pdf/model/optimize/image_ppi.go	56.71% <0%> (-16.42%)	⬇️
pdf/model/reader.go	50.5% <0%> (-15.94%)	⬇️
pdf/core/parser.go	66.53% <0%> (-15.12%)	⬇️
pdf/ps/exec.go	62.96% <0%> (-14.82%)	⬇️
pdf/core/encoding.go	44.28% <0%> (-13.98%)	⬇️
pdf/ps/operations.go	53.45% <0%> (-13.84%)	⬇️
pdf/model/signature_handler.go	55.04% <0%> (-13.77%)	⬇️
pdf/core/stream.go	31.08% <0%> (-13.52%)	⬇️
... and 94 more

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