Make crate no_std compatible

[Weypare]

This PR makes this crate no_std compatible.

Unfortunately, this comes with a breaking change - the msrv is bumped from 1.60 to 1.81, because of core::error::Error.
In case this is not acceptable, this can be mitigated either by adding a std feature and only implementing std::error::Error when the feature enabled or by using the approach similar to what is used in anyhow crate ^{1 2}.

Footnotes

1. https://github.com/dtolnay/anyhow/blob/master/build.rs#L73 ↩

2. https://github.com/dtolnay/anyhow/blob/master/src/lib.rs#L276-L280 ↩