[Data products] Add symbolic constants for record sizes

[bocchino]

In the generated code, add a symbolic constant for the size of each fixed-size record. That will allow users to size containers in terms of the record sizes, which is more convenient than using the underlying types. For example, the user can say "give me a container that's big enough to hold 10 records R" instead of reasoning that record R has struct type S and saying "give me a container that's big enough to hold 10 pairs of (record id, type S)."

We can also add constexpr functions for array records, e.g., "give me the size of an array record R with 10 array elements."

• Update generated code and unit tests
• Update FPP functional tests
• Update SignalGen example in Ref deployment
• Update F Prime design doc