io: Add class IFile

This will be the base class for all types of file streams (ReadFile,
WriteFile, CopyFile)
It the future, probably, it will become descendant of IStream
io: Add IWriteFIle

Interface for writing data into file streams.
io: Fix inaccuracy in description of IReadFile::read and IWriteFile::write

Description of return value says '0' on success, however, that is not
true. Now it is corrected to say '>0'.