Proposal: Add ability to specify execution queue in EffectRouter

[jeppes]

This adds the ability to specify which queue an effect should be handled on in the EffectRouter with the following syntax:

EffectRouter<Effect, Event>()
    .routeCase(...)
    .on(.main) // <--- execute this effect on the main thread
    .to { /* some UI action */ }

Previously, this would have required writing:

EffectRouter<Effect, Event>()
    .routeCase(...)
    .to { 
        DispatchQueue.main.async { 
            // Some UI action
        }
    }

This can safely be used in both raw loops and in MobiusController loops.

@JensAyton @pettermahlen

[jeppes]

This is currently returning Self, which means that .on(..) can be called multiple times on the same route.

This could be solved by adding another intermediate type. I'm not sure this is worth it.

We could also crash if a queue has already been set. Again, I think the risk of this is quite low, but we can discuss this further here in the comments.

[JensAyton]

I think this is fine

[codecov]

Codecov Report

Merging #155 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #155      +/-   ##
==========================================
+ Coverage   94.47%   94.50%   +0.02%     
==========================================
  Files          46       46              
  Lines        1284     1291       +7     
==========================================
+ Hits         1213     1220       +7     
  Misses         71       71              

Flag	Coverage Δ
#ios	94.50% <100.00%> (+0.02%)	⬆️
#macspm	94.44% <100.00%> (+0.03%)	⬆️

Impacted Files	Coverage Δ
...obiusCore/Source/EffectHandlers/EffectRouter.swift	98.24% <100.00%> (+0.24%)	⬆️

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