Clean up the includes in the IR folder

[fruffy]

Major rework of how the ir libraries are structured. This removes most of the includes from the ir.h file and distributes them across the compiler. There are two reasons why this is done.

1. The way the ir includes were previously structured was very brittle. Just changing includes slightly broke the compiler. This clashed with clang-format, which tried to sort includes accordingly to the pre-defined include order. Also, the IR cpp files had to be compiled in a specific order. This makes changes or refactoring much harder than necessary.
2. Every time ir.h was included, basically all library headers and ir headers were included. This likely causes an increase in compile time.

Now with these changes it should be much easier to make changes to the ir classes and files, including simplifying includes and dependencies.

This PR should definitely be tested on downstream extensions since it touches a core part of the compiler.

[mihaibudiu]

You should not mix reformatting with this PR, because it makes it very difficult to review.
The ir.h including everything was by design.
You should talk to @ChrisDodd about this change.

[vhavel]

Thanks for this, I agree it's better in long-run to maintain proper includes instead of having one "super-header". It's surprising that the amount of includes this PR needs to add is relatively low - headers are likely pulled in anyway - on the other hand it's great backends won't need many changes after this PR.

[vhavel]

Headers in frontends/p4 usually put names into namespace P4, should we do the same here?

[fruffy]

Yeah, most of the back ends and the mid end just use IR nodes and visitors. The generated IR file also still pulls in a lot of includes. I think this can also be improved, but at least the file is self-contained now.