Various CE3 improvements

[dangerousben]

Main changes:

• Use the SyncGenerators provided by the CE3 testkit to generate interesting Rerunnable instances
• Catbird-native unsafeRun (just using Await for now)
• Use the twitter-util provided Time and Stopwatch for the Clock methods

Divided up into small commits so should be easy enough to pull out anything that's contentious.

@felixbr this could go in before or after #267, I'm not sure what that's currently waiting on?

[felixbr]

I've merged my branch, so you can base your PR(s) on master without being blocked by me.

[codecov-commenter]

Codecov Report

Merging #347 (57b83f3) into master (4c92e11) will decrease coverage by 1.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #347      +/-   ##
==========================================
- Coverage   91.12%   90.10%   -1.03%     
==========================================
  Files          17       17              
  Lines         293      293              
  Branches        1        1              
==========================================
- Hits          267      264       -3     
- Misses         26       29       +3     

Impacted Files	Coverage Δ
...a/io/catbird/util/effect/RerunnableInstances.scala	100.00% <100.00%> (ø)
...rc/main/scala/io/catbird/util/effect/package.scala	9.09% <0.00%> (-45.46%)	⬇️
...il/src/main/scala/io/catbird/util/Rerunnable.scala	92.68% <0.00%> (+2.43%)	⬆️

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[DuncanHills]

Hi @dangerousben do you need any help getting this merged? I have a catbird-dependent project that I'd like to upgrade to CE3 soon and I'm taking an interest in improving this project.

[felixbr]

@DuncanHills I don't have time to review this but if you could do a quick review I'd be happy to merge this in. I honestly lost track of it, sorry @dangerousben