rkyv serialization doesn't work on Tree

[ureeves]

Describe the bug
Serializing a Tree using rkyv is currently impossible.

To Reproduce

let tree = Tree::new();
const SCRATCH_SIZE: usize = 256;
rkyv::to_bytes<_, SCRATCH_SIZE>(&tree); // This will fail to compile

Expected behaviour
The above should compile.

Additional context
This is due to the fact that rkyv does not support RefCell currently.