Macro empty_trace

macro_rules! empty_trace {
    ($t:path) => { ... };
    ($first:path, $($rest:path),+) => { ... };
}

Implements a no-op Trace for a type.

This will cause memory leaks if it is used to implement tracing on a type which ends up participating in a cycle. This is useful for types that are e.g. used as a key in std::collections::HashMap, but are not actually Gc pointers.