Delete description support

[kornelski]

Rust has deprecated Error::description and started warning against using it.

This PR removes support for description() in a backwards-compatible way. Descriptions won't be generated, and existing description definitions are no-ops.

[tailhook]

As we also need #44 and most projects think of bumping minimum rust version as a breaking change. Should we just delete description altogether and make a breaking release?

[kornelski]

Why not both?

This change is backwards-compatible, so it could be released straight away. And then the remaining code can be cleaned up for the next release.

[tailhook]

Hm. Yes that makes sense.

[tailhook]

Released as v1.2.3

[tailhook]

Is the fact that it changes default Display implementation should be considered as a breaking change?

[tailhook]

Also, this breaks any rust that has no default implementation for description() method. Which is somewhere between 1.24 an 1.31. Not sure if I should yank v1.2.3 now...

[kornelski]

It is more problematic than I assumed. I didn't consider old Rust versions, since I don't support them in any of my crates.

Maybe these changes aren't breaking in practice?