refactor(rust): More polars-io cleanup

[stinodego]

Changes

• Moved some tests to the integration test folder
• Clean up some imports/exports

[codecov]

Codecov Report

Attention: Patch coverage is 84.87805% with 31 lines in your changes are missing coverage. Please review.

Project coverage is 81.25%. Comparing base (8ea75df) to head (0db1ff0).
Report is 8 commits behind head on main.

Files	Patch %	Lines
crates/polars-io/src/shared.rs	68.42%	18 Missing ⚠️
crates/polars-io/src/partition.rs	0.00%	11 Missing ⚠️
crates/polars-io/src/ipc/mmap.rs	96.42%	1 Missing ⚠️
crates/polars-io/src/utils.rs	96.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15885      +/-   ##
==========================================
- Coverage   81.34%   81.25%   -0.09%     
==========================================
  Files        1384     1381       -3     
  Lines      176907   176716     -191     
  Branches     3033     3033              
==========================================
- Hits       143897   143591     -306     
- Misses      32525    32639     +114     
- Partials      485      486       +1     

Flag	Coverage Δ
python	74.70% <78.94%> (-0.04%)	⬇️
rust	78.38% <84.87%> (-0.09%)	⬇️

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