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?

阅读 SD 和 Bridge 库中现有的 File 定义，然后比较相关的 Print、Stream、Client 和 Server 基类。定义共享 API，并验证提供文件的库可以使用它，而无需消费者包含 SD.h；该 issue 未指定测试或具体的入口点。

由索引模型根据 Issue 内容生成。