Creating tarball with large files is slow

[Wojtek Mach]

I've narrowed it down to our custom code which makes in-memory tarball [1] (as opposed to always saving to disk like erl_tar does), so it's a matter of optimizing that or, more practical, creating temp file as before.

[1] https://github.com/hexpm/hex_erl/blob/d79a4615f947a9ce6deeeb97c3c4f9e52976be4f/src/hex_tarball.erl#L327:L348