Use core and alloc a bit more

[linkmauve]

We still depend mostly on std::io and std::fs, but the rest can be done without std.

[fintelia]

Could you say a bit about the motivation for this PR? The remaining usages of std are pretty fundamental to the crate, so I don't think we'll likely be able to switch to nostd any time soon (and at some point zune-jpeg will probably replace most use of this crate)

[linkmauve]

I’m upstreaming some of the changes I had to do to run this JPEG decoder on a GameCube in no_std. The rest of the changes are somewhat similar to #226, except I didn’t attempt to stay compatible with std::io.

I can switch to zune-jpeg if this is the more forward-looking crate.

[fintelia]

I just clarified maintenance status in the README. This crate isn't going anywhere, but going forward any big changes should focus on zune-jpeg