Provide a std::io::Write strftime implementation

[x-hgg-x]

I think the Error::source() implementation of strftime::Error is not necessary, since we can already match the error and extract the inner value in all cases.

[lopopolo]

I think the Error::source() implementation of strftime::Error is not necessary, since we can already match the error and extract the inner value in all cases.

The Error::source implementation is not for when you have a strftime::Error, but when you have a Box<dyn Error> that happens to be a strftime::Error.

I think we should also add the Error::IoError case to the existing source impl.

To provide some more context, a crate like eyre collects dyn Errors into a report type and reports the chain of error causes: https://docs.rs/eyre/latest/eyre/. This is only possible if we implement the Error::source method.

[x-hgg-x]

There is one more missed line since I removed derive implementations for strftime::Error. The Code Coverage check is not required, so this should not block the merge.