Add optional #[no_std] support

[Robbepop]

The types provided by apint could be also available using libcore only.
Maybe we would need to cross-out some features but all in all it should be possible.

This issue proposes to add the std crate feature.
It is supposed to be enabled by default: default = ["std"]
Users can opt-in to use libcore instead of standard lib by importing apint without default features:

apint = { version = "0.2", default-features = false }

In this issue we can track which APIs shall be available to #[no_std] scenarios.

[AaronKutch]

I think in between the crate reorganization PR and the unchecked_internals PR I will do this and maybe fix up the error type too. Should we release a new version right after the reorganization PR or do we want to wait?

[AaronKutch]

Or I could rebase the crate reorganization PR commits to include this since it is messing with imports anyway?

[Robbepop]

Or I could rebase the crate reorganization PR commits to include this since it is messing with imports anyway?

let us please do that in a separate PR.
as I have said I really want to keep PRs as small as possible for better reviewability and this one is already giantic.

I think it would be best to wait a bit for a release after the dust of all the new implementations and refactorings has settled down. Or what do you think?

[AaronKutch]

I think it would be best to wait a bit for a release after the dust of all the new implementations and refactorings has settled down. Or what do you think?
Yes this is what I think

[Robbepop]

Implemented by #50.

[Robbepop]

Implemented. Closed.