[P4Testgen] Resolve method call arguments before stepping into an extern - preserve InOut references

[fruffy]

Finally fixing this longstanding issue. We always had to resolve method call arguments within an extern, which often resulted in unnecessary redundant code. Further, for InOut parameters, the original reference was lost.

Now we frontload this directly in the method call expression. We resolve arguments In and InOut parameters before stepping into the extern. For InOut arguments, we replace the expression with an InOutReference, which preserves the reference. This way we can read the input argument and also write to the original reference once we step into an extern. With #4215 merged, we can also pass in any complex expression such as header or struct expressions as arguments.

[vlstill]

I like the way this simplifies writting extern implementations. I have some small questions and suggestions.