refactor(rust!): rename memmap -> memory_map as like Python

[eitsupi]

When reading an Arrow IPC file, the following warning message may appear, but it may be unclear what it means because the Rust API uses memmap instead of memory_map.

polars/crates/polars-io/src/ipc/ipc_file.rs

Line 84 in 7341aee

Toggle off 'memory_map' to silence this warning."

This PR updates memmap arguments to memory_map

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 81.30%. Comparing base (7341aee) to head (963e01c).

❗ Current head 963e01c differs from pull request most recent head 7b8c83e. Consider uploading reports for the commit 7b8c83e to get more accurate results

Files	Patch %	Lines
crates/polars-io/src/ipc/ipc_file.rs	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15642      +/-   ##
==========================================
- Coverage   81.30%   81.30%   -0.01%     
==========================================
  Files        1373     1373              
  Lines      176243   176243              
  Branches     2544     2544              
==========================================
- Hits       143293   143286       -7     
- Misses      32466    32475       +9     
+ Partials      484      482       -2     

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[codspeed-hq]

CodSpeed Performance Report

Merging #15642 will degrade performances by 11.74%

_{Comparing eitsupi:memory_map-in-rust (7b8c83e) with main (4eef5b5)}

Summary

❌ 1 regressions
✅ 21 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	eitsupi:memory_map-in-rust	Change
❌	test_sort_nan_1942	1 ms	1.2 ms	-11.74%