Add SwiftPM support (#1339)
Summary:
Using yoga in Swift
```swift
import UIKit
import yoga
let YGUndefined = Float.nan
UIGraphicsBeginImageContextWithOptions(CGSize(width: 1, height: 1), true, 0)
let scale = UIGraphicsGetCurrentContext()?.ctm.a ?? 1
UIGraphicsEndImageContext()
let config = YGConfigNew()
YGConfigSetUseWebDefaults(config, true)
YGConfigSetPointScaleFactor(config, Float(scale))
let root = YGNodeNewWithConfig(config)
YGNodeStyleSetFlexDirection(root, YGFlexDirectionRow)
YGNodeStyleSetFlexWrap(root, YGWrapWrap)
YGNodeStyleSetWidth(root, 200)
for i in 0..<4 {
let leaf = YGNodeNewWithConfig(config)
YGNodeStyleSetAspectRatio(leaf, 1)
YGNodeStyleSetWidthPercent(leaf, 50)
YGNodeInsertChild(root, leaf, UInt32(i))
}
YGNodeCalculateLayout(root, YGUndefined, YGUndefined, YGNodeStyleGetDirection(root))
for i in 0..<4 {
let leaf = YGNodeGetChild(root, UInt32(i))
let left = YGNodeLayoutGetLeft(leaf)
let top = YGNodeLayoutGetTop(leaf)
let width = YGNodeLayoutGetWidth(leaf)
let height = YGNodeLayoutGetHeight(leaf)
print("leaf \(i): origin: \(left), \(top), size: \(width), \(height)")
}
let width = YGNodeLayoutGetWidth(root)
let height = YGNodeLayoutGetHeight(root)
print("root size: \(width), \(height)")
YGNodeFreeRecursive(root)
YGConfigFree(config)
```
Pull Request resolved: https://github.com/facebook/yoga/pull/1339
Reviewed By: cipolleschi
Differential Revision: D48166958
Pulled By: NickGerleman
fbshipit-source-id: 72fef99729d01c7ba291b11f047cf75fd39f7630
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