Converting big images is slow

[iaguis]

I did some benchmarks with big images after #33 and the conversion is around 40% slower. This is so because we're compressing each layer and decompressing it when we squash to compress the squashed image again.

I think generating one ACI for each layer is a rather uncommon use case so compressing only the squashed image would be acceptable.

We could also consider using https://github.com/klauspost/pgzip to make it even faster.

[jonboulle]

How about just compressing the squashed image unless people are explicitly requesting an ACI for each layer? Or maybe that's what you mean...

[iaguis]

I wasn't meaning that but it makes total sense, silly me :)