Implemented Clone for AST nodes

[Michael-F-Bryan]

When implementing the code for rewriting parts of an AST, I noticed none of the AST types implement Clone.

Not having the impl means f you don't need to update a node and just want a copy of it, you have to implement the clone logic yourself instead of just writing node.clone(),

Is there any major reason why we don't #[derive(Clone)]?

[zbraniecki]

I incorporated it into https://github.com/projectfluent/fluent-rs/pull/198