feat(json): reduce string int decoding allocations

[tdakkota]

Benchstat comparison:

name                 old time/op    new time/op    delta
DecodeStringInt32-4    96.3ns ± 2%    55.0ns ± 1%   -42.87%  (p=0.000 n=14+15)
DecodeStringInt64-4    97.1ns ± 2%    52.5ns ± 1%   -45.89%  (p=0.000 n=14+15)

name                 old alloc/op   new alloc/op   delta
DecodeStringInt32-4     16.0B ± 0%      0.0B       -100.00%  (p=0.000 n=15+15)
DecodeStringInt64-4     16.0B ± 0%      0.0B       -100.00%  (p=0.000 n=15+15)

name                 old allocs/op  new allocs/op  delta
DecodeStringInt32-4      1.00 ± 0%      0.00       -100.00%  (p=0.000 n=15+15)
DecodeStringInt64-4      1.00 ± 0%      0.00       -100.00%  (p=0.000 n=15+15)

[codecov]

Codecov Report

Merging #319 (1b30743) into main (ba7472d) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #319      +/-   ##
==========================================
- Coverage   67.89%   67.85%   -0.04%     
==========================================
  Files         119      119              
  Lines        8029     8021       -8     
==========================================
- Hits         5451     5443       -8     
  Misses       2234     2234              
  Partials      344      344              

Impacted Files	Coverage Δ
json/int.go	100.00% <100.00%> (ø)

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