Deny unknown fields on serde structs

[schneems]

Add:

#[serde(deny_unknown_fields)]

To all structs that implement deserialize. For example:

#[derive(Deserialize, Serialize, Debug)]
#[serde(deny_unknown_fields)]
struct PersonV2 {
    name: String,
    updated_at: DateTime<Utc>
}

More info: https://github.com/schneems/magic_migrate/blob/3e43a72c329a2f03c016e0d37cd8ec085af7982f/README.md