Add Support for Deserializing newtype struct

[brettjnorris]

This resolves an issue I discovered where deserializing a newtype struct throws an error.

Given:

...

#[derive(Serialize, Deserialize)]
struct Id(u8);

export! {
    fn deserialize_id(id: Id) -> Id {
        id
    }
}

Calling the function throws this error:

Error: Error(Msg("invalid type: integer `1`, expected tuple struct Id"), State { next_error: None, backtrace: InternalBacktrace { backtrace: None } })

This change here resolves that error by adding a function to explicitly deserialize the newtype struct instead of passing to deserialize_any. I'm open feedback on whether this is the correct approach or if this can be improved on.

[oeed]

I can confirm I had the same issue and this does indeed solve it.