Make Tracer class less visible

The Tracer class currently resides in the AST package along side node classes, where it could collide with node names.

A possible solution would be to move the Tracer class into ASTNode or ASTNode$State. Alternatively, it could be renamed to ASTNode$Tracer. It should still be farily easy to access the tracer after the move. If the new name is long and unintuitive it might be a good idea to add an access method on ASTNode, or similar.