Support custom format on item for `Debug` derive macro

[ModProg]

I.e. something like:

#[derive(Debug)]
#[debug("{name}={value:?}")]
struct Arg {
  name: String,
  value: String
}

[JelteF]

Does this still not work with v1.0.0-beta.2?

[ModProg]

AFAICT it only works with Display

[tyranron]

@JelteF we have this on fields level for Debug, but missing on item level.

[mightyiam]

Thank you.