Serializing manifest result in TOML error

[antiphoton]

🐛 Bug report

Describe the bug

When trying to serialize wrangler::settings::toml::Manifest with some non-none HashMap value, the TOML serializer throws ValueAfterTable error. This is because TOML requires that table fields must come after other fields.

Reproduce the bug

The following code panics.

let manifest = Manifest {
    vars: Some(vec![
        (String::from("FOO"), String::from("some value")),
    ].into_iter().collect()),
    ..Default::default()
};
println!("{}", toml::to_string(&manifest).unwrap());

Expected behavior

The manifest is correctly serialized.

Environment and versions

• output of wrangler -V:
  1.19.0