Add persistence support for macOS

[StevenLambion]

• Added native support of macOS to the SwiftExtras module.
• Added macOS as a testing target.
• Updated formatting based on swift-format changes.

[codecov-io]

Codecov Report

Merging #16 into master will decrease coverage by 0.15%. The diff coverage is 70%.

@@            Coverage Diff             @@
##           master      #16      +/-   ##
==========================================
- Coverage   59.74%   59.58%   -0.16%     
==========================================
  Files          15       15              
  Lines         390      386       -4     
==========================================
- Hits          233      230       -3     
+ Misses        157      156       -1

Impacted Files	Coverage Δ
...DuxExtras/Persistence/PersistStateMiddleware.swift	0% <0%> (ø)	:arrow_up:
Sources/SwiftDux/Action/ActionPlan.swift	100% <100%> (ø)	:arrow_up:
Sources/SwiftDux/State/OrderedState.swift	81.45% <57.14%> (+0.41%)	:arrow_up:
Sources/SwiftDux/Store/Store.swift	94.11% <88.88%> (-0.17%)	:arrow_down:

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