[serlib] Explore ppx_override

[ejgallego]

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Since the beginning, the manual override / re-export of Coq's type using ppx_import has been done manually [with the help of some hacky scripting]. It was planned to use a more methodical method in the future.

It turns out such as solution by @thierry-martinez exists, https://github.com/thierry-martinez/override should allow for a much more compact derivation of serializes for Coq's types.