Minor simplifications and code dupe reductions in codegen

[Robbepop]

Carved out work from #665.

[codecov-commenter]

Codecov Report

Merging #938 (02a940c) into master (882b4dd) will decrease coverage by 0.16%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #938      +/-   ##
==========================================
- Coverage   82.84%   82.67%   -0.17%     
==========================================
  Files         177      177              
  Lines        8066     8058       -8     
==========================================
- Hits         6682     6662      -20     
- Misses       1384     1396      +12     

Impacted Files	Coverage Δ
crates/lang/codegen/src/generator/item_impls.rs	98.92% <100.00%> (+4.63%)	⬆️
crates/lang/ir/src/ir/item_impl/callable.rs	93.75% <100.00%> (+0.27%)	⬆️
...tes/storage/src/collections/bitstash/fuzz_tests.rs	40.00% <0.00%> (-60.00%)	⬇️
...rates/storage/src/collections/binary_heap/tests.rs	90.00% <0.00%> (-1.43%)	⬇️
...ates/storage/src/collections/hashmap/fuzz_tests.rs	98.93% <0.00%> (-1.07%)	⬇️

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