Implement MulAssign<T> for Mat and MatMul

[DJDuque]

Same as PR https://github.com/sarah-ek/faer-rs/pull/48, I am not sure if this is the correct way of implementing these traits. Nonetheless, I have been playing around with faer a lot lately, and having these traits has helped me to prototype and test stuff easily.

If this (or PR https://github.com/sarah-ek/faer-rs/pull/48) is something that you don't want in faer-core it is OK to close this as "wont fix".

[sarah-quinones]

the implementation looks good to me. if the CI runs successfully then i'll be merging this. (it's fine if it complains about code coverage. it does that sometimes)

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.48 :tada:

Comparison is base (000506c) 92.18% compared to head (eeb7005) 92.67%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #49 +/- ## ========================================== + Coverage 92.18% 92.67% +0.48% ========================================== Files 49 49 Lines 51597 51618 +21 ========================================== + Hits 47566 47837 +271 + Misses 4031 3781 -250 ``` | [Impacted Files](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/49?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/49?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah#diff-ZmFlci1jb3JlL3NyYy9saWIucnM=) | `75.20% <100.00%> (+6.89%)` | :arrow_up: | ... and [4 files with indirect coverage changes](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/49/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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