Rework the code building model constructors to leverage `Function` class
and dynamically emit a constructor function named after the model.
Before this change, all model classes were called "ModelConstructor",
which made debugging difficult.
After this change, a model class for model "User" is called "User.
Because not all valid model names are also valid JavaScript identifiers,
we implement a simple sanitization technique (replacing characters like
"-", "." and ":" with underscore "_") and fall back to legacy
"ModelConstructor" if the model name is still not a valid JS identifier.
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