implement Display for enums with attribute config_type

[scampi]

Implementing Display was motivated by the default value of edition being Edition2015 although it should have been 2015. Also, it provides a to_string method which could be used in #3620.

Below are the differences this PR makes on the output of --help=config --unstable-features:

137c137
<                      edition [2015|2018] Default: Edition2015
---
>                      edition [2015|2018] Default: 2015
164c164
<             required_version <string> Default: "1.3.0" (unstable)
---
>             required_version <string> Default: 1.3.0 (unstable)
191c191
<                       ignore [<string>,..] Default: IgnoreList { path_set: {}, rustfmt_toml_path: "" } (unstable)
---
>                       ignore [<string>,..] Default: [] (unstable)

[topecongiro]

Awesome, thank you!