Fix generated Error::provide and tests to reflect changes in API

[MegaBluejay]

Synopsis

Update derive and tests to work on the latest nightly with the changes in this PR.

Solution

Use error::{Request, request_ref, request_value} instead of any::{Demand, request_ref, request_value}

Currently request_value is not re-exported from core::error in std::error, which means backtrace tests require #![feature(error_in_core)]

Presumably this is a bug, and will be fixed later

Checklist

• Documentation is updated (if required)
• Tests are added/updated (if required)
• CHANGELOG entry is added (if required)

[tyranron]

@MegaBluejay nice job! Thanks 🍻