Suggest existing lint name in unknown_lints lint suggestion

[phansch]

It would be nice if the unknown_lints lint could suggest the correct lint name if a minor typo has been made.

For example using #![warn(dead_cod) should suggest dead_code.

Current output:

Compiling playground v0.0.1 (file:///playground)
warning: unknown lint: `dead_cod`
 --> src/main.rs:1:10
  |
1 | #![allow(dead_cod)]
  |          ^^^^^^^^
  |
  = note: #[warn(unknown_lints)] on by default

Expected output:

Compiling playground v0.0.1 (file:///playground)
warning: unknown lint: `dead_cod`
 --> src/main.rs:1:10
  |
1 | #![allow(dead_cod)]
  |          ^^^^^^^^ did you mean: `dead_code`?
  |
  = note: #[warn(unknown_lints)] on by default

[phansch]

I would like to work on this, too