AVRO-3248 Rust: Support named types in UnionSchema

[martin-g]

Drop the index map in UnionSchema. It is not common to have many schemata in the union. Usually it is either "null"+otherType or just a few of otherType. Iterating over a short list is not that slow

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[lulitao1997]

Sorry, i missied your message, somehow it did't send the notification by email :P

I checked and ur solution seems to be wrong if we have a union like this

["A", "B"]

where A and B are like this --

        // A and B are the same except the name.
        let schema_str_1 = r#"{
            "name": "A",
            "type": "record",
            "fields": [
                {"name": "field_one", "type": "float"}
            ]
        }"#;

        let schema_str_2 = r#"{
            "name": "B",
            "type": "record",
            "fields": [
                {"name": "field_one", "type": "float"}
            ]
        }"#;

in this case, the schema found by a value will always be "A"'s schema.

this also applies to A, B being "fixed" type, which seems plausible in reallife (for example A = {"name": "fieldA", "type": "fixed", "size": 10}, B = {"name": "fieldB", "type": "fixed", "size": 10})

here is my PR for these scenario . #1396

to solve that, we can change Value::Union to (index in the type list, value it holds), Similiar to Value::Enum.
this allows us to get Union's inner_schema for named types
directly, without validating the schema.

[martin-g]

Closed in favour of #1396