Emit error if Allsorts is built without one of the flate2 features

[wezm]

Fixes #53

If Allsorts is built without one of the flate2 features enabled emit a compile error with a message indicating this. The flate2 dependency is also marked optional so that in this case we a avoid a bunch of cryptic errors from trying to build that crate without a backend. There are still errors originating from Allsorts attempting to use flate2 when it's not been pulled in but there's fewer of them and they're a bit more obviously tied to the flate2 dependency.

[LoganDark]

LGTM!

[wezm]

For future reference this is what the errors look like:

[LoganDark]

Have you tested what happens when they are both enabled? It should just work then, right?

[wezm]

Have you tested what happens when they are both enabled?

Yes, flate2 builds ok. It appears to use zlib in that case

[wezm]

Merged manually.