refactor: Improve safety of amortized_iter

[ritchie46]

Removes a ton of unsafe tags and make UnstableSeries (now AmortSeries) cloneable. If cloned the Rc will be larger than 1 and the iterator will invalidate it's series_container and will create a new one. This almost never happens, currently only 1 test hits that.

If not the AmortSeries, but the underlying Series is cloned we also notice that by checking the underlying Arc and invalidating the iterators local container.

@itamarst FYI.

[codecov]

Codecov Report

Attention: Patch coverage is 97.82609% with 4 lines in your changes missing coverage. Please review.

Project coverage is 81.38%. Comparing base (38149d6) to head (7abb871).

Files	Patch %	Lines
crates/polars-plan/src/dsl/function_expr/list.rs	89.28%	3 Missing ⚠️
crates/polars-expr/src/expressions/mod.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16820      +/-   ##
==========================================
- Coverage   81.39%   81.38%   -0.02%     
==========================================
  Files        1424     1424              
  Lines      187623   187594      -29     
  Branches     2691     2691              
==========================================
- Hits       152719   152672      -47     
- Misses      34409    34427      +18     
  Partials      495      495              

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[codspeed-hq]

CodSpeed Performance Report

Merging #16820 will improve performances by 20.45%

_{Comparing unstable (7abb871) with main (38149d6)}

Summary

⚡ 1 improvements
✅ 36 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	unstable	Change
⚡	test_groupby_h2oai_q7	2.5 ms	2 ms	+20.45%