Move Visitor and Transformer out of ast.py

[stasm]

I'd like to consider making ast.py strictly about AST nodes. The processing classes could be moved to another module: tools.py, visitor.py, process.py or something else.

[Pike]

I briefly glanced at this, and then recognized that this is a breaking API change. Currently, consumers import them from ast, and if we tried to continue to expose them there, stuff would break :-(.

But yeah, we can do this in a new release of fluent.syntax.

[stasm]

Yep, that's why I filed a new issue about this, to discuss this independently of #139.