Added a more comprehensive Middleware API

[StevenLambion]

Work Performed

• Changed Middleware to a protocol type.
• Added HandleActionMiddleware for quick use cases such as debugging or testing.
• Added TypedMiddleware for middleware that require a known state type at initialization.

[codecov-io]

Codecov Report

Merging #25 into master will increase coverage by 4.98%. The diff coverage is 77.55%.

@@            Coverage Diff             @@
##           master      #25      +/-   ##
==========================================
+ Coverage   72.59%   77.57%   +4.98%     
==========================================
  Files          16       19       +3     
  Lines         467      486      +19     
==========================================
+ Hits          339      377      +38     
+ Misses        128      109      -19

Impacted Files	Coverage Δ
Sources/SwiftDux/Store/StoreProxy.swift	100% <ø> (+14.28%)	:arrow_up:
Sources/SwiftDux/Middleware/Middleware.swift	100% <100%> (ø)
Sources/SwiftDux/Store/Store.swift	94% <100%> (ø)	:arrow_up:
...s/SwiftDux/Middleware/HandleActionMiddleware.swift	100% <100%> (ø)
Sources/SwiftDux/Middleware/TypedMiddleware.swift	44.44% <44.44%> (ø)
...DuxExtras/Persistence/PersistStateMiddleware.swift	79.16% <79.16%> (+79.16%)	:arrow_up:
Sources/SwiftDuxExtras/PrintActionMiddleware.swift	70% <87.5%> (-7.78%)	:arrow_down:
... and 3 more

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