Improve the Middleware and Reducer pipeline

[StevenLambion]

Work Performed

• Consolidated chaining methods for actions.
• Removed next(_:) method from StoreProxy.
• Middleware now returns an optional Action instead of calling next(_:) off of the StoreProxy.
• Added the ReducerMiddleware so that the reducing step can be a part of the middleware pipeline.
• Fixed a circular dependency in the StoreProxy given to middleware.
• Replaced the asynchronous init of ActionPlan with one that returns a publisher to simplify the code.
• Removed the completion handler from ActionSubscriber.
• Fixed a bug introduced in iOS 14 that caused a crash related to the onReceive view modifier.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (2.x@0deffd3). Click here to learn what that means.
The diff coverage is n/a.

@@          Coverage Diff           @@
##             2.x      #52   +/-   ##
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  Coverage       ?   80.26%           
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  Files          ?       25           
  Lines          ?      456           
  Branches       ?        0           
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  Hits           ?      366           
  Misses         ?       90           
  Partials       ?        0           

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