This works very similarly to MarkedVector<T>, but instead of expecting
T to be Value or a GC-allocated pointer type, T can be anything.
Every pointer-sized value in the vector's storage will be checked during
conservative root scanning.
In other words, this allows you to put something like this in a
ConservativeVector<Foo> and it will be protected from GC:
struct Foo {
i64 number;
Value some_value;
GCPtr<Object> some_object;
};
