Add Submodules with simpler APIs to Pp_ast

[NathanReb]

This adds a functor and a Default module, similar to what's done in Ast_builder. One can use the functor to get pre-configured formatters or use the formatters provided by the Default module which are preconfigured with the default configuration.

This provides modules whose pretty printers can be used directly with "%a" format strings as they don't expect an optional argument first.