swift4.0遍历key有点麻烦，请问有没有更好的方法？
数据存储方式为：
let sourceDic: [string: [string]] = [
"Asia":["China", "Japan"],
"Africa":["Egypt", "Morocco"]
]
使用方式为，key为table.sectionName, value 为section.cell 显示内容。
在cellForRow方法中代码如下
let key = self.sourceDic.keys[indexPath.section]
let list = self.sourceDic[key]
let text = list![indexPath.row]
let cell = tableView.dequeueReusableCell(withIdentifier: cellIdentifier)
cell?.textLabel?.text = text
得到编译Error:
Cannot subscript a value of type 'Dictionary<String, [String]>.Keys' with an index of type 'Int'
意思是，不能对Dictionary<String, [String]>类型使用Int下标，然而在我的认知里，keys是数组而不是Dictionary。
查Dictionary.keys API，发现果然返回是字典，懵逼脸。
@available(swift 4.0)
public var keys: Dictionary<Key, Value>.Keys { get }
附带有用法说明：
/// let countryCodes = ["BR": "Brazil", "GH": "Ghana", "JP": "Japan"]
/// print(countryCodes)
/// // Prints "["BR": "Brazil", "JP": "Japan", "GH": "Ghana"]"
///
/// for k in countryCodes.keys {
/// print(k)
/// }
/// // Prints "BR"
/// // Prints "JP"
/// // Prints "GH"
然而我并不需要如此遍历，很不方便。
进一步查询官方教程 Apple Inc. “The Swift Programming Language (Swift 4)”
给出例子如下：
“var airports: [String: String] = ["YYZ": "Toronto Pearson", "DUB": "Dublin"]”
“let airportCodes = [String](airports.keys)
// airportCodes is ["YYZ", "LHR"]
let airportNames = [String](airports.values)”
// airportNames is ["Toronto Pearson", "London Heathrow"]
摘录来自: Apple Inc. “The Swift Programming Language (Swift 4)”
所以，必须使用强转喽？于是代码变成如下模样，说好swift的优雅呢？
let key = ([String](self.sourceDic.keys))[indexPath.section]
let list = self.sourceDic[key]
let text = list![indexPath.row]
