Implement the standard library Error trait when "std" is enabled

[faern]

Without having this trait implemented, a lot of existing error handling code will refuse to work with this type.

My current use case is I have an error that encapsulates KyberError and I want to be able to return it from my own std::error::Error::source implementation. But I can't, because the signature for that method requires the source to of course also implement std::error::Error.

[mberry]

The macOS test runner has a random lottery failure mode, all good on re-run.