Parse big ints using PyLong_FromString instead of BigInt

[samuelcolvin]

Seems to be a little bit slower, I thought this might be faster as it's what ujson does.

@davidhewitt any thoughts? Most of the time seems to be in PyLong_FromString, flamegraph here.

[codecov]

Codecov Report

Merging #61 (e184aa3) into main (13070b9) will decrease coverage by 0.28%.
The diff coverage is 91.54%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #61      +/-   ##
==========================================
- Coverage   95.35%   95.07%   -0.28%     
==========================================
  Files           8        8              
  Lines        1097     1137      +40     
==========================================
+ Hits         1046     1081      +35     
- Misses         51       56       +5     

Files	Coverage Δ
src/parse.rs	98.74% <100.00%> (ø)
src/python.rs	94.01% <100.00%> (+0.32%)	⬆️
src/number_decoder.rs	89.79% <90.00%> (-0.73%)	⬇️

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[codspeed-hq]

CodSpeed Performance Report

Merging #61 will improve performances by 16.86%

_{Comparing big-int-str (e184aa3) with main (13070b9)}

Summary

⚡ 3 improvements
✅ 48 untouched benchmarks

🆕 1 new benchmarks

Benchmarks breakdown

	Benchmark	main	big-int-str	Change
⚡	massive_ints_array_jiter_value	5.3 ms	4.7 ms	+12.66%
⚡	massive_ints_array_jiter_iter	4.3 ms	3.7 ms	+16.86%
⚡	bigints_array_jiter_iter	834.8 µs	744.4 µs	+12.14%
🆕	python_parse_massive_ints_array	N/A	7.6 ms	N/A

[samuelcolvin]

closing as this wasn't faster for strings, we'll use #65 instead.