Extend String to print 64-bit integers

`String(long long int)` and `String(unsigned long long int)` are not defined for relevant platforms, so `String()` is unable to parse 64-bit integers. A minimal non-working example would be:

```cpp
#include

void setup() {
String(1LL); // String(1L) compiles
}

void loop() {
}
```

Which produces an error `call of overloaded 'String(long long int)' is ambiguous`.

An open pull request (#277) already addresses this issue, so review it first. Start from the String overloads used through Arduino.h and compare the existing long-integer behavior with the missing 64-bit cases; the minimal example is done when String(1LL) compiles without an ambiguous-overload error.

Written by the indexing model from the issue text.