optional no std/rayon

[CGMossa]

I would vote for enabling rayon by default, even though that is odd... 🤷‍♂️

But either way is cool!

[sarah-ek]

rayon is enabled by default. the last commit was just to disable global parallelism in no-rayon builds

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (22daf13) 91.36% compared to head (2aa751c) 91.42%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #73 +/- ## ========================================== + Coverage 91.36% 91.42% +0.05% ========================================== Files 56 56 Lines 60626 60613 -13 ========================================== + Hits 55393 55413 +20 + Misses 5233 5200 -33 ``` | [Files](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/73?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah) | Coverage Δ | | |---|---|---| | [faer-cholesky/src/ldlt\_diagonal/compute.rs](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/73?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah#diff-ZmFlci1jaG9sZXNreS9zcmMvbGRsdF9kaWFnb25hbC9jb21wdXRlLnJz) | `92.90% <ø> (ø)` | | | [faer-cholesky/src/ldlt\_diagonal/solve.rs](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/73?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah#diff-ZmFlci1jaG9sZXNreS9zcmMvbGRsdF9kaWFnb25hbC9zb2x2ZS5ycw==) | `29.31% <ø> (ø)` | | | [faer-cholesky/src/ldlt\_diagonal/update.rs](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/73?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah#diff-ZmFlci1jaG9sZXNreS9zcmMvbGRsdF9kaWFnb25hbC91cGRhdGUucnM=) | `88.58% <ø> (ø)` | | | [faer-cholesky/src/lib.rs](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/73?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah#diff-ZmFlci1jaG9sZXNreS9zcmMvbGliLnJz) | `0.00% <ø> (ø)` | | | [faer-cholesky/src/llt/compute.rs](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/73?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah#diff-ZmFlci1jaG9sZXNreS9zcmMvbGx0L2NvbXB1dGUucnM=) | `92.77% <ø> (ø)` | | | [faer-cholesky/src/llt/inverse.rs](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/73?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah#diff-ZmFlci1jaG9sZXNreS9zcmMvbGx0L2ludmVyc2UucnM=) | `75.40% <ø> (ø)` | | | [faer-cholesky/src/llt/reconstruct.rs](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/73?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah#diff-ZmFlci1jaG9sZXNreS9zcmMvbGx0L3JlY29uc3RydWN0LnJz) | `59.52% <ø> (ø)` | | | [faer-cholesky/src/llt/solve.rs](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/73?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah#diff-ZmFlci1jaG9sZXNreS9zcmMvbGx0L3NvbHZlLnJz) | `34.57% <ø> (ø)` | | | [faer-cholesky/src/llt/update.rs](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/73?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah#diff-ZmFlci1jaG9sZXNreS9zcmMvbGx0L3VwZGF0ZS5ycw==) | `87.53% <ø> (ø)` | | | [faer-core/src/householder.rs](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/73?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah#diff-ZmFlci1jb3JlL3NyYy9ob3VzZWhvbGRlci5ycw==) | `89.97% <ø> (ø)` | | | ... and [32 more](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/73?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah) | |

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.