Add an optional feature for 1.34.2 backwards compatibility

[HeroicKatora]

The readme states that the crate is compatible with 1.34, however this is no longer true due to relying on extern crate alloc which was stabilized in 1.36. Since alloc is just a subset of std we can restore this compatibility via an optional feature, disabled by default and fully opt-in.

Alternatives

• Add a features to disable all alloc-dependent items. This isn't really feasible with the current design which relies on Box and Vec internally.
• Don't do this. Then image might not profit from potential bugfixes until 0.24 :)

@alexcrichton Would this be fine and compatible with std inclusion, by not activating the feature there?

[alexcrichton]

This should work for libstd, yes, but I'd probably recommend to instead just bump the minimum version.

[HeroicKatora]

We had promised not to do that without a major version. Well, it's been almost a year and there are a few potentially breaking changes in the pipeline anyways. I'll consider this another reason to do it.

[oyvindln]

1.34 is what's in debian stable, so I think it may be reasonable to still be compatible with it, if it can be done without causing other issues.

[oyvindln]

Would people be okay with merging this? I see there is some demand for being able to update.

[Alphare]

I have commented in #90 the reasons why I'm in favor of this PR being merged.