Expose a way to decompress "partial" RAW_DEFLATE data, that doesn't have lastblock bit set

[sgmihai]

As previously talked here https://forum.nim-lang.org/t/12495 (thanks for constructive criticism btw).
I patched proc inflate by adding `and (b.len - b.pos != 0) in the while loop condition, and not it works.
But it would be nice to have this functionality exposed in some way. It seems nntp servers treat the whole connection, not each message, as a stream, and only send the lastBlock bit set on disconnection, so you need to decompress data in the meantime.
I'd rather use zippy than zlib for no external dependencies.