[P4Testgen] Reduce the use of static objects in P4Testgen

[fruffy]

CompilerTarget, Target, and Context are all singleton, static mystery objects floating around the interpreter. They are initialized in a specific way that makes it hard to make changes or extend the tool, in particular it is difficult to make P4Testgen a library because these static objects always need be initialized properly.

Find a way to reduce the usage of these objects.

[vlstill]

Re #4334 (comment), as I think this is more relevant there:

Yup, there are a bunch of these singletons scattered around and several utility functions depend on it. I believe they are largely used to keep track of errors and warnings.

Hmm, for errors it will be quite tricky to get rid of them, unless we want every place that can throw an error to carry a ref to the context :-(. That could be not too hard for passes, but there are a lot of errors/warning emitted outside passes (e.g. constant overflows, etc.). But in some way it could be prettier if the diagnostics were not everywhere, but mostly in passes. An alternative would be to create all the objects through/with some sort of context (I think LLVM does it that way at least for types, or at least did when I was using it).