feat: Support Decimal read from IPC

[nameexhaustion]

Resolves #15920

128-bit values could be unaligned, in which case we need to do a copy (write_ipc currently doesn't align them).

[codecov]

Codecov Report

Attention: Patch coverage is 93.22034% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 80.89%. Comparing base (c9e786b) to head (0ef7b2c).

Files	Patch %	Lines
crates/polars-arrow/src/mmap/array.rs	94.82%	3 Missing ⚠️
crates/polars-arrow/src/array/mod.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15965      +/-   ##
==========================================
+ Coverage   74.36%   80.89%   +6.53%     
==========================================
  Files        1367     1384      +17     
  Lines      169193   178047    +8854     
  Branches     3043     3043              
==========================================
+ Hits       125818   144036   +18218     
+ Misses      42892    33527    -9365     
- Partials      483      484       +1     

Flag	Coverage Δ
python	74.37% <93.22%> (+<0.01%)	⬆️
rust	78.03% <93.22%> (+6.68%)	⬆️

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[nameexhaustion]

Coverage got ratelimited

error - 2024-04-30 09:29:27,115 -- Commit creating failed: {"detail":"Tokenless has reached GitHub rate limit. Please upload using a token: https://docs.codecov.com/docs/adding-the-codecov-token. Expected available in 339 seconds."}


[ritchie46]

I think we can use a tuple directly. E.g. (A, B).

[nameexhaustion]

I like that idea, a lot :D

[codspeed-hq]

CodSpeed Performance Report

Merging #15965 will improve performances by 17.13%

_{Comparing nameexhaustion:ipc (0ef7b2c) with main (c9e786b)}

Summary

⚡ 1 improvements
✅ 34 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	nameexhaustion:ipc	Change
⚡	test_tpch_q15	7.8 ms	6.7 ms	+17.13%