File base class

Similar to Print, Stream, Client, Server... we probably need a File base class.

Currently File is defined in the SD library, and also in the Bridge library for Yun. Most programs in the Arduino ecosystem which use files are tied to the SD library. Because they include SD.h in their headers, the SD library is automatically brought into the project.

But the SD library isn't the only possible source for files. Flash chips, USB mass storage, network filesystems can also provide files. Without a base class, each library using files needs to be edited.

Use of alternate SD card libraries, like SdFat, is also very difficult without a base class because so many libraries which consume files are SD.h included by their code.

Hopefully the time has come to move File out of the SD library and into an abstract base class?

Lies die bestehenden File-Definitionen in den SD- und Bridge-Bibliotheken und vergleiche anschließend die zugehörigen Basisklassen Print, Stream, Client und Server. Definiere die gemeinsame API und überprüfe, dass Bibliotheken, die Dateien bereitstellen, sie verwenden können, ohne dass Consumer SD.h einbinden müssen; das Issue nennt keinen Test und keinen spezifischen Einstiegspunkt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.