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.

[patricoferris]

This looks fine to me.

Is there any benefit to doing this the Ast_builder way fully by extracting the type interface and using a 'a with_loc and 'a without_location type (except for with and without a config) ?

[NathanReb]

Yes that sounds like a good idea! I'll factor out the pretty-printer type!

I'll go ahead and make the test suite 5.3 compatible first though!

[NathanReb]

I pushed the changes with the new alias types. I didn't go for with/without_config as I found those names a bit ambiguous. One could read with_config either as with a config argument or with an embedded configuration. I used configured and configurable instead as I found those would leave less room for interpretation.