re-order untagged enum

[Farossco]

When parsing the following type of schema definition:

{
    "definitions": {
        "Value": {
            "type": [
                "array",
                "boolean",
                "integer",
                "number",
                "object",
                "string"
            ]
        }
    }
}

The following rust type is generated:

pub enum Value {
    Boolean(bool),
    Object(::serde_json::Map<::std::string::String, ::serde_json::Value>),
    Array(::std::vec::Vec<::serde_json::Value>),
    Number(f64),
    String(::std::string::String),
    Integer(i64),
}

This is mostly what we want but the issue here is the ordering. The use of a BTreeMap not only deduplicates but also re-orders the types to follow the ordering of the InstanceType enum from schemars which puts the Number type before the Integer type.

This makes the Integer variant of our Value enum here unreachable as all possible values can already be matched by the Number variant.

I believe that forcing the re-ordering of the variant instead of following the ordering declared in the schema is a good thing but it should be slightly tweaked.

This PR introduces an intermediate re-definition of this InstanceType enum with re-ordered fields and uses it for this BTreeMap deduplication.

This in terms, gives us the following rust enum definition:

pub enum Value {
    Boolean(bool),
    Integer(i64),
    Number(f64),
    String(::std::string::String),
    Array(::std::vec::Vec<::serde_json::Value>),
    Object(::serde_json::Map<::std::string::String, ::serde_json::Value>),
}