Allow overriding built-in type names

[Kriskras99]

As mentioned by @KitFieldhouse in #476 (comment) this test should pass:

#[test]
fn avro_rs_508_allow_overriding_type_names() -> TestResult {
    let schemas = Schema::parse_list([
    r#"{
        "type": "record",
        "name": "fixed",
        "fields": [{
            "name": "foo",
            "type": "string"
        }]
    }"#,
    r#"{
        "type": "record",
        "name": "Spam",
        "fields": [{
            "name": "bar",
            "type": "fixed"
        }]
    }"#
    ])?;

    Ok(())
}