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?

Read the existing File definitions in the SD and Bridge libraries, then compare the related Print, Stream, Client, and Server base classes. Define the shared API and verify that file-providing libraries can use it without consumers including SD.h; the issue does not name a test or specific entry-point.

Written by the indexing model from the issue text.