Separate Error and Display derives

[SCingolani]

I was faced with the same as this issue: #250

I have a no_std project but I want to use both the Display and From implementations from this crate.

This is still a work in progress, pending:

• Add / update documentation
• Some attributes do nothing for Display (e.g. source) so this should give an error?

Would there be any opposition for such a change? As per commented in the linked issue, this could be a nice to have until stabilization of error_in_core.

[dtolnay]

I commented on the issue — this is not something I would want to support in this crate.

Thanks anyway for the PR!