Add support for Subtle and DER.

[recmo]

Motivation

Makes it easier to use UInts for implementing cryptographic protocols.

Solution

PR Checklist

• Added Tests
• Add support for conversion from der::asn1 any/int formats
• Added Documentation
• Updated the changelog

[prestwich]

let me know when this is ready for review 👍

[recmo]

@prestwich Ready for review!

[prestwich]

mostly no_std compilation related changes

[recmo]

@prestwich Please review!

I did look briefly in adding a 'does it build in no_std' step to the CI, but it seems that as of today there are still no good solutions to test this. But the clippy lint I added should cover the majority of problems.

[prestwich]

yeah, we are currently relying on the cargo hack CI step to do a large number of --no-default-features recompilations and it is definitely not sufficient

last outstanding item is to qualify or import Box in der.rs. then this looks good to go

cargo check --no-default-features --features der

[prestwich]

🙌