Clean up parsing of multi-member PDFs

[Radonirinaunimi]

Addresses #46 by keeping rayon but with some clean up.

[codecov]

Codecov Report

❌ Patch coverage is 18.86792% with 86 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.37%. Comparing base (b854e5e) to head (54c5935).
⚠️ Report is 15 commits behind head on master.

Files with missing lines	Patch %	Lines
neopdf/src/pdf.rs	5.55%	34 Missing ⚠️
neopdf_capi/src/lib.rs	0.00%	32 Missing ⚠️
neopdf_pyapi/src/pdf.rs	32.00%	17 Missing ⚠️
neopdf/src/parser.rs	70.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
- Coverage   85.18%   83.37%   -1.82%     
==========================================
  Files          25       25              
  Lines        3793     3891      +98     
==========================================
+ Hits         3231     3244      +13     
- Misses        562      647      +85     

Flag	Coverage Δ
capi	52.09% <11.11%> (-1.41%)	⬇️
python	52.41% <13.51%> (-1.09%)	⬇️
rust	86.67% <25.00%> (-0.47%)	⬇️

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