Special case remote = "Self"

[dtolnay]

The following has worked since remote was introduced. It generates fn serialize and fn deserialize with the same signature as Serialize::serialize and Deserialize::deserialize.

#[derive(Serialize, Deserialize)]
#[serde(remote = "RemoteSelf")]
struct RemoteSelf;

This PR makes the following equivalent to the above. This did not work before due to our use of the remote path in places where Self had not the right meaning.

#[derive(Serialize, Deserialize)]
#[serde(remote = "Self")]
struct RemoteSelf;