Expose total_in/total_out from underlying flate2 encoder types.

[tobz]

👋🏻

This PR is about exposing some more of the underlying flate2 bits, as the title indicates.

Naturally, when asynchronously writing the compressed output, it can be difficult, otherwise, to determine how much data has been written so far. By exposing the helper methods like total_in and total_out, it becomes that much easier.

Some thoughts/caveats:

• I do realize that this is somewhat coupling the API to flate2, although with flate2 being 1.x, it should be OK from a stability standpoint.
• Not all compression algorithms used come from flate2, so not all encoders can consistently expose these same methods.

[robjtede]

Exposing a useful -> u64 getter doesn't feel like it's exposing internals too much. I'd be comfortable committing to this API. Will wait for consensus before merging though.

[NobodyXu]

the API looks fine to me, exposing the total in/out as u64 seems reasonable for me.

Can't think of a reason to use another type, i.e. negative total in/out doesn't make sense, and total in/out should be always available.

[tobz]

Oh wow, awesome!

Thanks so much y'all.