refactor: make Trace an unsafe trait for boa_engine compatibility

[shruti2522]

updated the Trace trait in both null_collector_branded and mark_sweep_branded to be an unsafe trait so we can
integrate it into boa_engine

my initial thought was to keep it a safe trait, but doing so broke the engine's closure type inference. Trace must be an unsafe trait, because if an implementor forgets to trace a GC pointer, it causes a UAF bug that the compiler can't catch.

I also changed the method signature from trace(&mut self) to trace(&self). This is necessary because gc objects frequently form reference cycles.

• changed fn trace(&mut self) to unsafe fn trace(&self)
• upgraded all impl Trace blocks to unsafe impl Trace
• updated bounds to accept ?Sized types, added an as_ptr() helper to Gc

this ensures oscars is compatible with boa_engine, allowing to drop it in as a backend without changing any core engine code