nanoserde: fix compile error when using proxy as a struct annotation

[Adjective-Object]

It looks like proxying structs was originally intended to be implemented, but when I tried it out, it produced code that failed to compile with the following warning:

error[E0308]: mismatched types
   --> crates/nanoserde/tests/json.rs:960:14
    |
960 |     #[derive(DeJson, SerJson, PartialEq, Debug)]
    |              ^^^^^^
    |              |
    |              expected `&str`, found `&mut Chars<'_>`
    |              arguments to this function are incorrect
    |
    = note:      expected reference `&str`
            found mutable reference `&mut Chars<'_>`
note: associated function defined here
   --> /workspaces/macroquad-test/crates/nanoserde/src/serde_json.rs:82:8
    |
82  |     fn deserialize_json(input: &str) -> Result<Self, DeJsonErr> {
    |        ^^^^^^^^^^^^^^^^
    = note: this error originates in the derive macro `DeJson` (in Nightly builds, run with -Z macro-backtrace for more info)

This change fixes the issue, and adds a test to defend it against future breaks.

[knickish]

Thanks, this is a good catch