Add RuleConfig

[mcabbott]

This wants to add methods for rrule_via_ad and frule_via_ad from ChainRules.

Need to figure out how to test this; so far rrule_via_ad does seem to work. Not sure anything is in the right place.

[simeonschaub]

Testing should now be set up, so you'll probably want to rebase this.

[simeonschaub]

Would it be possible to add a test specifically making use of this?

[mcabbott]

Yes was just trying to figure out how (and where). I've added something, but my frule_via_ad definition is clearly completely wrong.

[codecov-commenter]

Codecov Report

Merging #45 (48460e7) into main (0a98045) will increase coverage by 0.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #45      +/-   ##
==========================================
+ Coverage   55.80%   55.98%   +0.17%     
==========================================
  Files          21       21              
  Lines        2100     2106       +6     
==========================================
+ Hits         1172     1179       +7     
+ Misses        928      927       -1     

Impacted Files	Coverage Δ
src/runtime.jl	58.33% <100.00%> (+3.78%)	⬆️
src/stage1/forward.jl	88.40% <100.00%> (+0.34%)	⬆️
src/stage1/generated.jl	76.30% <100.00%> (+0.22%)	⬆️
src/extra_rules.jl	42.10% <0.00%> (+0.43%)	⬆️

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[oxinabox]

You can test this using ChainRulesTestUtils if you wanted.
It supports testing these cases.

This basically LGTM.
I see no reason not to merge it once tests pass

[simeonschaub]

CI failure is JuliaLang/julia#42125, will push a fix shortly if no one beats me to it.

[simeonschaub]

Just as a small comment: I'd generally always squash-merge PRs like this unless the commits are actually standalone changes.

[mcabbott]

Sorry, will do, next time.