AD extension package

[lkdvos]

This moves the AD rules into a package extension. The implementation for linsolve comes from the current master The implementation for eigsolve comes from one of the open PRs (#56 )

TODO

• [ ] look into setting up tests in more uniform way
• [ ] use ChainRulesTestUtils
• [ ] add svdsolve AD rules
• [ ] add expsolve AD rules

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.13139% with 19 lines in your changes are missing coverage. Please review.

Project coverage is 82.34%. Comparing base (68b3baf) to head (bdf7a97). Report is 1 commits behind head on master.

Files	Patch %	Lines
ext/KrylovKitChainRulesCoreExt/eigsolve.jl	85.71%	19 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #83 +/- ## ========================================== + Coverage 82.05% 82.34% +0.29% ========================================== Files 27 29 +2 Lines 2753 2889 +136 ========================================== + Hits 2259 2379 +120 - Misses 494 510 +16 ```

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[Jutho]

Thanks for starting this. I would also still like to experiment with my alternative solution for doing eigsolve AD, which itself uses eigsolve instead of linsolve.

[lkdvos]

I think in any case we should look for a solution to determine how the rrule is computed, even just changing the tolerances should be accessible.

Also, I think @pbrehmer has an implementation for the svdsolve, so I'll tag him here :)