Factor convenience functions out of main printer implementation
The pretty printer in rustc_ast_pretty has a section of methods commented "Convenience functions to talk to the printer". This PR pulls those out to a separate module. This leaves pp.rs with only the minimal API that is core to the pretty printing algorithm.
I found this separation to be helpful in https://github.com/dtolnay/prettyplease because it makes clear when changes are adding some fundamental new capability to the pretty printer algorithm vs just making it more convenient to call some already existing functionality.
761705ebe5