Add printer for code #69

Merged kento.tngl.sh
targeting performance
Merged opened by and assigned to kento.tngl.sh
targeting performance

Hi! Thanks for all the hard work on Forester 😃

Pp_code aims to print the AST (Code.t) in such a way that parsing, printing and reparsing returns the same AST.

This is a much more complete version of what Graft has coded up which may be useful to other users of Forester's API.

The new test/roundtrip roundtrips all of the trees in docs/trees using the new Code.equal function to compare the first and second parses. Small disclaimer, I am not a very confident Formater, I'm sure there is plenty we could do to improve the output, but this is perhaps an ok first pass.

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