Fix warnings when using custom_trace macro from Rust 2024 Edition

[todesking]

When using custom_trace! in the Rust 2024 Edition, calling mark directly can trigger an unsafe_op_in_unsafe_fn warning. However, wrapping mark in an unsafe block causes an unused_unsafe warning instead. This PR modifies the macro so that no warnings are emitted when mark is called inside an unsafe block.

#[derive(gc_derive::Finalize)]
pub struct Foo;

unsafe impl gc::Trace for Foo {
    gc::custom_trace!(this, {
        // warning[E0133]: call to unsafe function `<runtime::Foo as gc::Trace>::unroot::mark` is unsafe and requires unsafe block
        mark(this);
    });
}

#[derive(gc_derive::Finalize)]
pub struct Foo;

unsafe impl gc::Trace for Foo {
    gc::custom_trace!(this,  {
        // warning: unnecessary `unsafe` block
        unsafe { mark(this); }
    });
}