String relational operators are members.

Member relational operators are no good because they cause the lhs and rhs to be treated differently. This causes problems such as making == appear non-commutative. Such as:

```c++
void setup() {
String("hi") == "world";
"world" == String("hi");
}
```

```
sketch_jan19a:3: error: no match for 'operator==' (operand types are 'const char [6]' and 'String')
"world" == String("hi");
^
exit status 1
```

It would be less user-surprising to bring these operators out of the class.

I propose defining them at namespace scope as symmetric pairs of operators, between a String on one side, and everything String accepts as an implicit conversion on the other side, per:
http://en.cppreference.com/w/cpp/string/basic_string/operator_cmp

使用 issue 中的两个 C++ 表达式作为首次复现，然后定位 ArduinoCore-API 中 String 的关系运算符声明。比较两种操作数顺序并检查现有的 String 行为；完成的标准是等价比较能够编译并以对称方式运行，同时覆盖报告的案例。

由索引模型根据 Issue 内容生成。