Feature request: add FrameDecoder.total_in() and .total_out() to return the number of compressed bytes and uncompressed_bytes read so far

[DeflateAwning]

Feature request: add the following methods:

1. FrameDecoder frame_decoder.total_in(): returns the number of compressed bytes which have entered the decompressor so far
2. FrameDecoder frame_decoder.total_out(): returns the number of decompressed bytes outputed by the compressor so far

These values appear to be stored easily as private variables in the FrameDecoder struct. It would be awesome if they were accessible.

Reasoning

1. The comparable XzDecoder in the xz2 library has this exact setup.
2. It would make creating progress report messages very straight-forward.

Other Notes

• Perhaps creating a TotalInOut trait for this would be cool?
• Ideally, ensure that reading these values does not require a mutable reference to the FrameDecoder. Seems obvious, but getting the current seek location in a File requires a mutable reference, so I guess it's not as obvious as you'd think.

[PSeitz]

I'd accept a pull request for that.

seek changes the state of File and is therefore mut.

[DeflateAwning]

In your opinion, should I create a trait for this, or just implement the functions? I'm pretty new to Rust, and don't have a great grasp on effective library design yet. I'll work on implementing this.

Re seek (which isn't relevent here) - file_reader.stream_position() requires a mutable reference because it calls self.seek(SeekFrom::Current(0)) under the hood. This call does not actually seek (i.e., it does not move the read "pointer"). It frustrates me.

[PSeitz]

I think just the functions should be fine