Make TextualFromNative output deterministic

Nobody has claimed this yet.

TextualFromNative output is not deterministic due to the underlying map type used for the data struct.
Ideally the method should be deterministic similarly to what the [json.Marshal](https://golang.org/pkg/encoding/json/#Marshal) already does by sorting the map keys.

Current behaviour:
```
codec, err := goavro.NewCodec(`
{
"type": "record",
"name": "Schema",
"fields" : [
{"name": "first", "type": "string"},
{"name": "second", "type": "string"}
]
}`)
if err != nil {
fmt.Println(err)
}

val := map[string]interface{}{"first":"foo", "second":"bar"}
for i:=0; i<20;i++ {
b, _ := codec.TextualFromNative(nil, val)
fmt.Println(string(b))
}
```

Result:
```
{"first":"foo","second":"bar"}
{"second":"bar","first":"foo"}
{"first":"foo","second":"bar"}
{"second":"bar","first":"foo"}
{"first":"foo","second":"bar"}
{"first":"foo","second":"bar"}
{"first":"foo","second":"bar"}
{"first":"foo","second":"bar"}
{"first":"foo","second":"bar"}
{"first":"foo","second":"bar"}
{"second":"bar","first":"foo"}
{"first":"foo","second":"bar"}
{"second":"bar","first":"foo"}
{"first":"foo","second":"bar"}
{"first":"foo","second":"bar"}
{"first":"foo","second":"bar"}
{"first":"foo","second":"bar"}
{"first":"foo","second":"bar"}
{"first":"foo","second":"bar"}
{"first":"foo","second":"bar"}
```

Start at codec.TextualFromNative, the entry point shown in the issue, and trace how map-backed record fields are emitted. Add a regression test that calls the method repeatedly with the sample value and verifies stable, key-sorted output; done means the output is deterministic like json.Marshal.

Written by the indexing model from the issue text.