Make std::io::Take<R> an instance of std::io::Seek when R:std::io::Seek

This would be particularly useful for reading file formats containing other formats.
Examples include:

• OpenType font files, where CFF is an embedded format.
• GRIB (format used by weather forecast organisms), containing grayscale JPEG to describe values.

(and probably many others)
In such cases, it would also be cool to have a "drain" method on std::io::Take, that would move the read cursor at the end of the Take.

[sfackler]

How would this implementation behave when SeekFrom::Start or SeekFrom::End is passed?

Well, for my particular use case, it would be most useful to see a "subfile", hence, if len is the length passed to take:

• take does a seek(SeekFrom::Current(0)) to determine the current position at the moment take(len) is called, let's call that position offset.
• then SeekFrom::Start(n) actually performs a SeekFrom::Start(offset+n) on the underlying Seek.
• SeekFrom::End(n) does SeekFrom::Start(offset + len + n) on the underlying Seek.

[dtolnay]

I am not 100% sold on the proposed implementation, but I would be interested in seeing a well-tested PR for adding this impl. I agree that seeking within a "subfile" seems reasonable.