Allow this crate to be used in no_std

[notgull]

Resolves #17 by implementing initial no_std support. This is a breaking change. I did not implement no_alloc because it would require a substantial change to the codebase.

[pyfisch]

This is a breaking change.

Is this only breaking because of the Cargo features, if you use no-default-features? Or does it actually require code changes?

[notgull]

Is this only breaking because of the Cargo features, if you use no-default-features? Or does it actually require code changes?

Without the std feature, certain error types no longer implement std::error::Error, which is technically a breaking change.

[pyfisch]

Postponed until no-std support is needed in a project.