[stdlib] Dictionary: Add implementation for first

This is primarily for documentation purposes, although the default implementation (based on an iterator) may not return the correct value for bridged dictionaries with exotic implementations.

Author: lorentey

stdlib/public/core/Dictionary.swift

@@ -707,6 +707,21 @@ extension Dictionary: Collection {
   public var isEmpty: Bool {
     return count == 0
   }
+
+  /// The first element of the dictionary.
+  ///
+  /// The first element of the dictionary is not necessarily the first element
+  /// added to the dictionary. Don't expect any particular ordering of
+  /// dictionary elements.
+  ///
+  /// If the dictionary is empty, the value of this property is `nil`.
+  @inlinable
+  public var first: Element? {
+    // FIXME: It'd better to use an iterator than to subscript with startIndex,
+    // because startIndex is currently O(n) in bridged dictionaries. However,
+    // enumerators aren't guaranteed to have the same element order as allKeys.
+    return count > 0 ? self[startIndex] : nil
+  }
 }
 
 extension Dictionary {

stdlib/public/core/Set.swift

@@ -384,6 +384,9 @@ extension Set: Collection {
   /// If the set is empty, the value of this property is `nil`.
   @inlinable
   public var first: Element? {
+    // FIXME: It'd better to use an iterator than to subscript with startIndex,
+    // because startIndex is currently O(n) in bridged sets. However,
+    // enumerators aren't guaranteed to have the same element order as allKeys.
     return count > 0 ? self[startIndex] : nil
   }
 }