Consider renaming Tranformers

[mctaverna]

The world transformer is also used to refer to a deep-learning model, introduced in 2017 by a team at Google.
It is used primarily in the fields of natural language processing (NPL) and it could generate confusion among the readers.

[alessiostalla]

Actually, they're called ASTTransformer in the source code, but we tend to refer to them as simply "transformers". I think we should make the effort to always call them "AST transformers" at least in the official documentation and examples, to avoid confusion with other kinds of transformers.