Allow the user the get the current number of bytes written to the wrapped writer.

[PaulGrandperrin]

In a project I'm working on, I need to know an estimation of how much bytes have already been written to decide when to call Encoder.finish().
I need that to be able to create evenly sized compressed blocks in a bigtable like implementation.

[bozaro]

I do not like this change:

• Adding Encoder.get_written_size also involves adding Decoder.get_readed_size;
• This functionality can be added without lz4-rs modification by code like:

struct Wrapper<W: Write> {
    w: W,
    written_size: usize,
}

impl<W: Write> Wrapper<W> {
    pub fn get_written_size(&self) -> usize {
        self.written_size
    }
}

impl<W: Write> Write for Wrapper<W> {
    fn write(&mut self, buffer: &[u8]) -> Result<usize> {
        self.written_size += buffer.len();
        self.w.write(buffer)
    }

    fn flush(&mut self) -> Result<()> {
        self.w.flush()
    }
}

[PaulGrandperrin]

I didn't thought of that but I think it won't work.
When you build an Encoder, the Writer is moved inside it and is therefore inaccessible until you get it back from finish().

As a matter of fact, the Writter I'm using already implements the method len() which I could have used if it wasn't moved...

[bozaro]

Sounds reasonable.
I will add this feature fore Encoder/Decoder in few days.

[PaulGrandperrin]

Great, thank you! I didn't felt like using the C bindings directly :-)