[P4Testgen] Add struct and header expressions as a first-class expression to P4Testgen.

[fruffy]

This PR adds support for struct expressions in P4Testgen. Concretely, it adds utility functions to handle assignments and pass struct expressions as full argument. It also generalizes some utility functions to support BaseListExpressions, StructExpressions, and HeaderStackExpressions.

The goal of these changes is to enable two features:

• Support of meta information of complex P4 objects (such as validity and the next index of header stacks)
• Support of struct expressions as method call arguments. This will make implementation of externs significantly easier because we do not have to resolve the arguments within the extern any longer. Instead, we are able to access all necessary arguments directly.

It also introduces a new IR type. HeaderExpressions. HeaderExpressions track the validity of the header as an expression, which makes it possible to track validity of a header symbolically and not just as a boolean constant.