perf: Use downcast_ref instead of dtype equality in <dyn SeriesTrait as AsRef<ChunkedArray<T>>

[coastalwhite]

This sped up concat by ~2x for me.

[codecov]

Codecov Report

Attention: Patch coverage is 23.43750% with 49 lines in your changes missing coverage. Please review.

Project coverage is 78.95%. Comparing base (92fd75d) to head (d681241).
Report is 31 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-core/src/series/mod.rs	44.44%	10 Missing ⚠️
...s/polars-core/src/series/implementations/binary.rs	0.00%	3 Missing ⚠️
...s-core/src/series/implementations/binary_offset.rs	0.00%	3 Missing ⚠️
.../polars-core/src/series/implementations/boolean.rs	0.00%	3 Missing ⚠️
...ars-core/src/series/implementations/categorical.rs	0.00%	3 Missing ⚠️
...tes/polars-core/src/series/implementations/date.rs	0.00%	3 Missing ⚠️
...polars-core/src/series/implementations/datetime.rs	0.00%	3 Missing ⚠️
.../polars-core/src/series/implementations/decimal.rs	0.00%	3 Missing ⚠️
...polars-core/src/series/implementations/duration.rs	0.00%	3 Missing ⚠️
...tes/polars-core/src/series/implementations/null.rs	0.00%	3 Missing ⚠️
... and 4 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20664      +/-   ##
==========================================
- Coverage   79.00%   78.95%   -0.05%     
==========================================
  Files        1566     1557       -9     
  Lines      221034   220778     -256     
  Branches     2510     2527      +17     
==========================================
- Hits       174618   174314     -304     
- Misses      45842    45886      +44     
- Partials      574      578       +4     

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