Use `mul!` in compute slope function

[codecov[bot]]

Codecov Report

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

Comparison is base (5419b18) 96.68% compared to head (46ee7c4) 96.70%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #192 +/- ## ========================================== + Coverage 96.68% 96.70% +0.01% ========================================== Files 7 8 +1 Lines 181 182 +1 ========================================== + Hits 175 176 +1 Misses 6 6 ``` | [Impacted Files](https://app.codecov.io/gh/JuliaSmoothOptimizers/SolverTools.jl/pull/192?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaSmoothOptimizers) | Coverage Δ | | |---|---|---| | [src/auxiliary/bounds.jl](https://app.codecov.io/gh/JuliaSmoothOptimizers/SolverTools.jl/pull/192?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaSmoothOptimizers#diff-c3JjL2F1eGlsaWFyeS9ib3VuZHMuamw=) | `97.50% <100.00%> (ø)` | | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/JuliaSmoothOptimizers/SolverTools.jl/pull/192/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaSmoothOptimizers)

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DCISolver.jl
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