Add an optional std feature that implements std::error::Error

[phil-opp]

The std::error::Error trait is required for some error handling frameworks. This pull request adds an optional std feature that implements it for the Error enum.

The new std feature is also useful if linreg is compiled together with other dependencies that also use the displaydoc crate, but with its default feature set. Since Cargo unifies all dependency features, displaydoc is compiled with its std feature enabled, even though linreg sets default-features = false for it. This results in a compilation failure if the std library is not included in linreg.