BufWriter: Provide into_raw_parts

If something goes wrong, one might want to unpeel the layers of nested
Writers to perform recovery actions on the underlying writer, or reuse
its resources.

`into_inner` can be used for this when the inner writer is still
working.  But when the inner writer is broken, and returning errors,
`into_inner` simply gives you the error from flush, and the same
`Bufwriter` back again.

Here I provide the necessary function, which I have chosen to call
`into_raw_parts`.

I had to do something with `panicked`.  Returning it to the caller as
a boolean seemed rather bare.  Throwing the buffered data away in this
situation also seems unfriendly: maybe the programmer knows something
about the underlying writer and can recover somehow.

So I went for a custom Error.  This may be overkill, but it does have
the nice property that a caller who actually wants to look at the
buffered data, rather than simply extracting the inner writer, will be
told by the type system if they forget to handle the panicked case.

If a caller doesn't need the buffer, it can just be discarded.  That
WriterPanicked is a newtype around Vec<u8> means that hopefully the
layouts of the Ok and Err variants can be very similar, with just a
boolean discriminant.  So this custom error type should compile down
to nearly no code.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

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