Local inner macros

[dtolnay]

@SergioBenitez I need to add a helper macro to json_internal! to pick up rust-lang/rust#51496 and enable:

#![feature(use_extern_macros)]

use serde_json::json;

How would you like me to proceed? If it is is easier for rocket_contrib we could add an unused json_internal_vec! in one release, re-export it from rocket_contrib, then begin using it from json_internal! in another release.