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?

SD および Bridge ライブラリにある既存の File 定義を読み、関連する Print、Stream、Client、Server 基底クラスを比較します。共有 API を定義し、ファイルを提供するライブラリが、利用側で SD.h をインクルードせずにそれを使用できることを検証します。この issue では、テストや特定のエントリーポイントは指定されていません。

索引モデルが issue の本文から書いたものです。