Migrate Codegen module to Java

[aman-dureja]

https://github.com/CategoricalData/hydra/blob/main/hydra-haskell/src/main/haskell/Hydra/Codegen.hs

[joshsh]

Note: this should be done eventually, but it is a P1 rather than a P0. The current SoT for all of Hydra's built-in models is Haskell, and that will continue to be the case for the foreseeable future. For third-party models which users want to define in Java, there is always the option of exporting the models to JSON (provided that the epsilon function and a JSON encoder are available in Java), then picking them up in Haskell to generate code.