Write pretty printer in terms of `('a, 'b) statement` and `'a expression` about stanc3 HOT 4 CLOSED

Currently, the code is specialized to the untyped ast.

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@seantalts , not sure I see how to do this. Wouldn't the be a row polymorphic function again? Not sure we can naturally write those in OCaml.

Note, though, that we can also pretty print a typed ast by first taking untyped_program_of_typed_program of the ast. I actually wonder if we could address your other use case for row polymorphism with a similar map from the decorated to the undecorated ast.

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We talked about this already in person, but for posterity / if someone else wants to tackle this:
we might be able to use a technique like the emit_statement_meta parameter here: https://github.com/stan-dev/stanc3/blob/master/lib/Stan_math_backend.ml#L179

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This is fixed, thanks to @palmerlao on #180.

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