Derived "apply" implementation for enums is broken

[Diggsey]

When diffing an enum like:

#[derive(SerdeDiff, Serialize, Deserialize, Debug, PartialEq, Clone)]
enum Value {
    Str(String),
    Int(i32),
}

Let's say we have the diff Value::Str("A") -> Value::Str("B").

The diff implementation will produce the following sequence of commands:

[{"Enter":{"EnumVariant":"Str"}},{"Enter":{"FieldIndex":0}},{"Value":"B"},"Exit"]

However, the apply implementation does not consume the final Exit command, causing the rest of the command stream to get out of sync.

This is because of this code: https://github.com/amethyst/serde-diff/blob/e461da41c9339af3ecadf9f4aa61ae91c1f79e97/serde-diff-derive/src/serde_diff/mod.rs#L362-L379

Specifically it's because next_path_element is only called once, instead of being called until it finds an Exit command. It makes sense to only call it once because an enum can only have one variant, but since the variant still counts as a path segment, an additional Exit should be consumed.