Can specify which corners of a `UIView` are rounded. Extended `CACornerMask` OptionSet to be a bit more readable and friendlier to use.

RoundCornersWithMaskInUIKit.swift

extension UIView {
    
    /// Applies rounded corners with masking to specific corners and with corner curve option.
    func roundCorners(cornerRadius: CGFloat, roundedStyle: CALayerCornerCurve, corners: CACornerMask) {
        layer.cornerRadius = cornerRadius
        layer.cornerCurve = roundedStyle
        layer.maskedCorners = corners
    }
}

extension CACornerMask {
    
    /// Applies mask to all corners
    public static var allCorners: CACornerMask {
        [.layerMaxXMaxYCorner, .layerMinXMinYCorner, .layerMaxXMinYCorner, .layerMinXMaxYCorner]
    }
    
    /// Applies mask to top left and top right corners
    public static var topCorners: CACornerMask {
        [.layerMinXMinYCorner, .layerMaxXMinYCorner]
    }
    
    /// Applies mask to bottom left and bottom right corners
    public static var bottomCorners: CACornerMask {
        [.layerMinXMaxYCorner, .layerMaxXMaxYCorner]
    }
    
    /// Applies mask to top left and bottom left corners
    public static var leftCorners: CACornerMask {
        [.layerMinXMinYCorner, .layerMinXMaxYCorner]
    }
        
    /// Applies mask to top right and bottom right corners
    public static var rightCorners: CACornerMask {
        [.layerMaxXMinYCorner, .layerMaxXMaxYCorner]
    }
}