replace heap allocations with stack allocated matrix in triangular multiplication

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01% :tada:

Comparison is base (e811fa8) 92.72% compared to head (075acd9) 92.74%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #57 +/- ## ========================================== + Coverage 92.72% 92.74% +0.01% ========================================== Files 49 49 Lines 52302 52341 +39 ========================================== + Hits 48497 48543 +46 + Misses 3805 3798 -7 ``` | [Files Changed](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/57?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah) | Coverage Δ | | |---|---|---| | [faer-core/src/lib.rs](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/57?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah#diff-ZmFlci1jb3JlL3NyYy9saWIucnM=) | `76.87% <ø> (+0.08%)` | :arrow_up: | | [faer-core/src/mul.rs](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/57?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah#diff-ZmFlci1jb3JlL3NyYy9tdWwucnM=) | `77.10% <100.00%> (+0.25%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/57/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah)

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