Everywhere: Rename to_{string => deprecated_string}() where applicable

This will make it easier to support both string types at the same time
while we convert code, and tracking down remaining uses.

One big exception is Value::to_string() in LibJS, where the name is
dictated by the ToString AO.

Userland/Games/GameOfLife/Pattern.cpp

@@ -30,7 +30,7 @@ void Pattern::rotate_clockwise()
         for (int j = m_pattern.size() - 1; j >= 0; j--) {
             builder.append(m_pattern.at(j).substring(i, 1));
         }
-        rotated.append(builder.to_string());
+        rotated.append(builder.to_deprecated_string());
     }
     m_pattern = move(rotated);
 }