Handling of maximum precision is inconsistent between decimal and scientific notation

[janlugt]

I ran into this issue when parsing JSON with floats that are sometimes written in scientific notation. When a float exceeds the maximum scale, it is simply cut off by rust_decimal when it's written in decimal notation, but in scientific notation, an error is generated. To reproduce:

#[derive(serde::Deserialize)]
struct F {
    f: rust_decimal::Decimal
}

// Simply parsing as a `serde_json::Value` works fine.
assert!(serde_json::from_str::<serde_json::Value>(
    r#"{ "f": 1E-29 }"#,
).is_ok());

// Parsing the same number in decimal notation to a `rust_decimal::Decimal` is fine too.
assert!(serde_json::from_str::<F>(
    r#"{ "f": 0.00000000000000000000000000001 }"#,
).is_ok());

// But parsing it when written in scientific notation fails.
assert!(serde_json::from_str::<F>(
    r#"{ "f": 1E-29 }"#,
).is_err());