http://stackoverflow.com/questions/29707622/bizarre-swift-compiler-error-expression-too-complex-on-a-string-concatenation
https://github.com/RobertGummesson/BuildTimeAnalyzer-for-Xcode
https://medium.com/swift-programming/speeding-up-slow-swift-build-times-922feeba5780#.81znje73y
https://medium.com/@RobertGummesson/regarding-swift-build-time-optimizations-fc92cdd91e31#.hlpji1e2t
https://medium.com/swift-programming/swift-build-time-optimizations-part-2-37b0a7514cbe#.rctp6s5r3
1.主要原因是type inference, 所以尽量不要让编译器做太多的类型推断.
1.1 避免 无类型字典中过多的 ??
1.2 避免复杂的+号表达式