Provide derive(Trace) support for structures to implement gcmodule::Trace interface.

Example

use gcmodule_derive::Trace;

#[derive(Trace)]
struct S<T: gcmodule::Trace> {
    a: String,
    b: Option<T>,

    #[trace(skip)] // ignore this field for Trace.
    c: MyType,
}

struct MyType;