Remove Debug nesting in errors that only contain an inner error

[dtolnay]

fn main() {
    println!("{:#?}", toml::from_str::<i32>("").unwrap_err());
}

Before:

Error {
    inner: Error {
        inner: TomlError {
            message: "invalid type: map, expected i32",
            raw: Some(
                "",
            ),
            keys: [],
            span: Some(
                0..0,
            ),
        },
    },
}

After:

TomlError {
    message: "invalid type: map, expected i32",
    raw: Some(
        "",
    ),
    keys: [],
    span: Some(
        0..0,
    ),
}

[coveralls]

Pull Request Test Coverage Report for Build 12324386302

Details

• 0 of 6 (0.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.07%) to 67.983%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
crates/toml/src/de.rs	0	2	0.0%
crates/toml/src/ser.rs	0	2	0.0%
crates/toml_edit/src/de/mod.rs	0	2	0.0%

Totals
Change from base Build 12146327906:	-0.07%
Covered Lines:	3752
Relevant Lines:	5519

---

💛 - Coveralls