Inherit the visibility of remote struct definition

[dtolnay]

For remote derive this will give the derived methods the same visibility as the definition.

#[serde(remote = "...")]
struct Def; // Def::serialize and deserialize are private to this module

#[serde(remote = "...")]
pub struct Def; // Def::serialize and deserialize are public

#[serde(remote = "...")]
pub(crate) struct Def; // Def::serialize and deserialize are pub(crate)

Fixes #1000. @arcnmx

[arcnmx]

Such a prompt response! Looks like a good solution to me.