Dealing with constants in `freeze`

Right now, I think we bake constants into the code no matter what, at considerable expense. (For example, we might recompile every time the time step changes.) Maybe defaulting the other way (replace every numerical constant with a variable) is better?

cc @kaushikcfd @matthiasdiener

Start by tracing the freeze implementation and how numerical constants are handled during code generation. Compare the current constant-baking behavior with replacing constants by variables, including the recompilation cost when the time step changes; done requires a decided strategy and corresponding validation.

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