build: Update Rust nightly toolchain version

[stinodego]

New lint catches nonexistent feature flags - which effectively leads to dead code in many cases. The fix is to either add the feature flag or remove the dead code.

There was quite a lot to fix. A few notable things:

• Removed the polars-arrow/src/util/bench_util.rs module, which is unused and hidden behind a nonexistent feature flag.
• Removed two Parquet test modules with test that weren't running and referenced non-existing code.
• Added feature flags for use_mimalloc and default_allocator for the PyO3 bindings.
• Some PyO3 code was hidden behind a non-existing flag, it had to be updated to the new bound API.

[codecov]

Codecov Report

Attention: Patch coverage is 10.52632% with 17 lines in your changes are missing coverage. Please review.

Project coverage is 80.86%. Comparing base (004efbe) to head (cde5618).
Report is 2 commits behind head on main.

Files	Patch %	Lines
py-polars/src/map/series.rs	0.00%	13 Missing ⚠️
crates/polars-arrow/src/datatypes/mod.rs	0.00%	1 Missing ⚠️
crates/polars-arrow/src/datatypes/physical_type.rs	0.00%	1 Missing ⚠️
crates/polars-arrow/src/types/mod.rs	0.00%	1 Missing ⚠️
crates/polars-json/src/json/write/serialize.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16222      +/-   ##
==========================================
- Coverage   80.93%   80.86%   -0.08%     
==========================================
  Files        1394     1395       +1     
  Lines      179564   179956     +392     
  Branches     2908     2909       +1     
==========================================
+ Hits       145329   145515     +186     
- Misses      33730    33936     +206     
  Partials      505      505              

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