Display: Add a prefix for all enum variants

[ErichDonGubler]

It'd be really nice to be able to derive Display and use an affix (prefix and/or suffix) that comes before each inner implementation of Display. This is particularly handy for errors; an example I'm hoping to work with might look like:

    #[derive(Debug, Display)]
    // Expects a single Display item corresponding to where an inner 
    // variant's Display will be written (`{}`), otherwise errors
    #[display(affix = "unable to read from /proc/stat: {}")]
    pub(crate) enum SnapshotReadError {
        Io(IoError),
        #[display(fmt = "expected CPU stat line for {}, got {:#?}", expected_cpu, line)]
        UnableToParseLine {
            expected_cpu: CpuComponent,
            line: String,
        }
    }

    #[derive(Debug, Display)]
    pub(crate) enum CpuComponent {
        #[display(fmt = "entire CPU")]
        EntireCpu,
        #[display(fmt = "CPU core {}", _0)]
        Core(usize),
    }

What do you think?

Please note: I specifically chose a terrible but concise word for this which I fully expect to be bikeshedded. Alternatives might include:

• outer_fmt (I like this one the best.)
• prefix (but it's not just a prefix!)
• surround

[ErichDonGubler]

Ping! I've implemented a PoC on my fork's affix branch. What do you think, @JelteF?

[JelteF]

Could you make a PR? Makes it much easier to review

[ErichDonGubler]

Sure! I'll close this in favor of #79.

The reason I didn't simply file a PR initially is because I hacked the support together to demonstrate via test/ what this would look like, and don't particularly like the commit as-is. I'll just mark the PR [WIP] and be patient when it comes time to clean things up. :)