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

Use the two C++ expressions in the issue as the first reproduction, then locate the String relational-operator declarations in ArduinoCore-API. Compare both operand orders and check the existing String behavior; done means equivalent comparisons compile and behave symmetrically, with coverage for the reported case.

Written by the indexing model from the issue text.