StringView: Make construction of String from a StringView containing a String cheaper

... at the cost of an additional pointer per view.

AK/AKString.h

@@ -36,9 +36,12 @@ public:
 
     String() {}
 
-    String(StringView view)
+    String(const StringView& view)
-        : m_impl(StringImpl::create(view.characters(), view.length()))
     {
+        if (view.m_string)
+            *this = String(*view.m_string);
+        else
+            m_impl = StringImpl::create(view.characters(), view.length());
     }
 
     String(const String& other)

AK/StringView.cpp

@@ -4,7 +4,8 @@
 namespace AK {
 
 StringView::StringView(const AK::String& string)
-    : m_characters(string.characters())
+    : m_string(&string)
+    , m_characters(string.characters())
     , m_length(string.length())
 {
 }

AK/StringView.h

@@ -44,6 +44,8 @@ public:
     bool operator==(const String&) const;
 
 private:
+    friend class String;
+    const AK::String* m_string { nullptr };
     const char* m_characters { nullptr };
     int m_length { 0 };
 };