Feature: Allow to skip seq access for internally and adjacently tagged enums

[Dzordzu]

Both the adjacently tagged and internally tagged representations can be
expressed either via map or sequence syntax. For example:

#[derive(Debug, Serialize, Deserialize)]
#[serde(tag = "type", content = "x")]
enum MyType {
    Topic(i64), Sidebar(i64)
}

fn main() {}

is equivalent to both (sequence syntax)

[ "Topic", 1 ]

and (map syntax)

{ "type": "Topic", "x": 1 }

Disable the sequence format

In case you want to disable the sequence format, you can use the
#[serde(seq=false)].

#[derive(Debug, Serialize, Deserialize)]
#[serde(tag = "type", content = "x", seq_form=false)]
enum MyType {
    Topic(i64), Sidebar(i64)
}

will casue the sequence syntax to be disabled.