Support for no_std environments

[QuinnWilton]

I know this isn't a priority, but I thought I'd get the issue down in writing for tracking purposes.

Summary

Problem

The crate is currently incompatible with no_std

Impact

The crate can't be used in embedded or bare-metal environments, like on a Raspberry Pi or ESP32.

Solution

I haven't done a comprehensive survey of what would be required to make rs-ucan compatible with no_std, but at minimum we must:

• use default-features = false for serde, and explicitly list derive as a feature
• Support using serde-json-core instead of serde_json
• Support using heapless in place of dynamically allocated types like Vec and String

There's likely more work that would be required than the above, especially to support both environments without a ton of duplicated code, but the above points would be good starting points for anyone investigating this!

[cdata]

This sounds like a great thing to have 👍