FunctionK instance for Rerunnable ~> Future

[dangerousben]

So tiny as to be arguably pointless but I've found I needed it more
than once.

[codecov-commenter]

Codecov Report

Merging #240 into master will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #240      +/-   ##
==========================================
+ Coverage   91.30%   91.35%   +0.05%     
==========================================
  Files           9        9              
  Lines         161      162       +1     
  Branches        3        5       +2     
==========================================
+ Hits          147      148       +1     
  Misses         14       14              

Impacted Files	Coverage Δ
...il/src/main/scala/io/catbird/util/Rerunnable.scala	90.36% <100.00%> (+0.11%)	⬆️

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[felixbr]

Looks good to me.

I have no idea what the naming convention for FunctionK instances is. I feel like toFutureK would be clearer, so I'll let @travisbrown merge this one :)

[travisbrown]

I like toFuture, especially given that in normal usage it's not clear that there's a FunctionK involved at all.