Port to last version of the decompress library

[rlepigre]

see ocaml/opam-repository#12313.

[cfcs]

This will require an explicit bound on either decompress <= 0.7.0 or (preferably) decompress >= 0.8.0 to prevent situations where we try to compile against the wrong version of decompress.

1. @rlpigre what do you think about including an opam file in the repository?
   This would allow easier linting and CI of imagelib while developing new patches.

2. I propose adding an imagelib.opam to do this, and ideally also a imagelib-unix.opam package at some point with the Unix-specific things (right now I believe imagelib stopped compiling on mirage again due to the Unix dependency I introduced when fixing the shell injection issue).

[rlepigre]

@cfcs OK, I'm fine with that.

[vbgl]

May this issue be fixed? It would be nice to be able to use “imagelib” with latest “decompress”.

It seems that there is a couple of pull requests that are ready to be merged. Thanks.

[cfcs]

I think #23 is ready to be merged, and then we can address @rymdhund's comments in a separate PR.
The reason for not updating that PR is that I have a lot of branches based on #23 that would be tricky to rebase (GIF support, test suite written with @olleolleolle, image scaling, ...).

[rlepigre]

@cfcs feel free to merge #23, that looks good to me.