Use abseil maps even more

[asl]

This is PR on top of #4459 to show how we can improve the things even more with better data structures.

Essentially, this uses more efficient data structures on some hot code paths:

• abseil hash maps / sets in reference resolver and type map
• abseil maps / sets & pre-allocated vector in use-def analisys
• remove some useless allocations and memory traffic in use def

Here are results:

Command	Mean [s]	Min [s]	Max [s]	Relative
gtestp4c-abseil --gtest_filter=P4CParserUnroll.switch_20160512	6.056 ± 0.232	5.773	6.465	1.29 ± 0.05
gtestp4c --gtest_filter=P4CParserUnroll.switch_20160512	4.701 ± 0.061	4.600	4.783	1.00

gtestp4c-abseil is #4459 (so yes 78% faster a total compared to main).

[asl]

I think some other benchmarks are also required. Unfortunately, we do not have much that could be ported from downstream to "stock" p4c.

[vlstill]

I went over the C++ changes. This definitely goes in the right direction, thank you. But I have some points that I believe needs to be addressed. You seem to be using Util::Hash only sometimes, and it is not clear to me if this is deliberate, or if it is an omission. Also, I suggest we make the get availble for the abseil maps to simplify both migration and the code itself.

[fruffy]

Actually, can we make absl a bracketed system include to be consistent?

[asl]

I think someone asked somewhere exactly the opposite for fetched dependencies :)

[fruffy]

Really? I believe all our fetched dependencies (protobuf, gtest, inja, z3) are included using bracketed includes. Can you point me to the discussion?

[asl]

Need to find. But I'm happy to change to angled brackets. This makes much more sense to me :)

[asl]

Ok, apparently abseil expects to use quoted includes. And bazel build even enforces this (the path is exported as -iquoted).

[fruffy]

Ugh, seems to be an opinionated maintainer. This might cause issues with Werror and clang-tidy. Not a big deal but annoying.

We can just patch this, I can get this done in a separate PR.

[fruffy]

Approving from my side.