Improve lifting of "data" expressions

There are instances where Chronikis-generated Stan code has subexpressions being computed in the model section that could be computed in the transformed data section. This should be fixed so that they are computed once only, in transformed data.

No file, test, or entry point is named in the issue. Trace the compiler pass that lifts data expressions into generated Stan code, then inspect representative generated models to verify that eligible subexpressions are emitted once in transformed data rather than recomputed in model.

Written by the indexing model from the issue text.