Reduce Swift compile time.

[frankrausch]

I profiled Kingfisher’s compile time after reading this:
http://irace.me/swift-profiling

With this small change, I was able to speed up compile time dramatically.

Before:

2517.2ms  Kingfisher/Sources/String+MD5.swift:213:10  func calculate() -> [UInt8]
2444.7ms  Kingfisher/Sources/String+MD5.swift:282:20  (closure)
2320.0ms  Kingfisher/Sources/String+MD5.swift:213:10  func calculate() -> [UInt8]
2251.4ms  Kingfisher/Sources/String+MD5.swift:282:20  (closure)
113.8ms Kingfisher/Sources/String+MD5.swift:129:6 func toUInt32Array(_ slice: ArraySlice<UInt8>) -> Array<UInt32>
108.5ms Kingfisher/Sources/String+MD5.swift:129:6 func toUInt32Array(_ slice: ArraySlice<UInt8>) -> Array<UInt32>
47.5ms  Kingfisher/Sources/String+MD5.swift:63:6  func arrayOfBytes<T>(_ value: T, length: Int? = default) -> [UInt8]
46.4ms  Kingfisher/Sources/String+MD5.swift:63:6  func arrayOfBytes<T>(_ value: T, length: Int? = default) -> [UInt8]
39.8ms  Kingfisher/Sources/String+MD5.swift:69:86 (closure)
39.3ms  Kingfisher/Sources/String+MD5.swift:69:86 (closure)
…

After:

123.0ms Kingfisher/Sources/String+MD5.swift:213:10  func calculate() -> [UInt8]
103.6ms Kingfisher/Sources/String+MD5.swift:129:6 func toUInt32Array(_ slice: ArraySlice<UInt8>) -> Array<UInt32>
101.5ms Kingfisher/Sources/String+MD5.swift:129:6 func toUInt32Array(_ slice: ArraySlice<UInt8>) -> Array<UInt32>
73.1ms  Kingfisher/Sources/String+MD5.swift:213:10  func calculate() -> [UInt8]
45.3ms  Kingfisher/Sources/String+MD5.swift:63:6  func arrayOfBytes<T>(_ value: T, length: Int? = default) -> [UInt8]
45.2ms  Kingfisher/Sources/String+MD5.swift:63:6  func arrayOfBytes<T>(_ value: T, length: Int? = default) -> [UInt8]
39.2ms  Kingfisher/Sources/String+MD5.swift:69:86 (closure)
38.4ms  Kingfisher/Sources/String+MD5.swift:69:86 (closure)
16.1ms  Kingfisher/Sources/String+MD5.swift:157:19  mutating func next() -> ArraySlice<UInt8>?
12.2ms  Kingfisher/Sources/String+MD5.swift:39:21 get {}
…

[codecov-io]

Current coverage is 73.57% (diff: 100%)

Merging #467 into master will increase coverage by 0.04%

@@             master       #467   diff @@
==========================================
  Files            21         21          
  Lines          2210       2214     +4   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits           1625       1629     +4   
  Misses          585        585          
  Partials          0          0          

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[onevcat]

Wow, incredible! Thanks for it.
I will also read that article later. Thanks for letting me know it!