Remove IR::Annotations and make IAnnotated to carry annotations inline

[asl]

Lots of cleanups and fixes here and there

[asl]

This is PR on top of https://github.com/p4lang/p4c/pull/4971 for simplicity.

As mentioned in https://github.com/p4lang/p4c/issues/4974, IR::Annotations incur a huge overhead, and in general does not make any sense: it's just another level of indirection. So, this just implements old FIXME, plus does some cleanup here and there.

The impact of # of allocations is huge. Before: After:

So, we're 10% less of memory allocations by both total size and number.

The runtime impact with GC disabled is huge as well:	Command	Mean [s]	Min [s]	Max [s]	Relative
test/gtestp4c-nogc-main --gtest_filter=P4CParserUnroll.switch_20160512	3.240 ± 0.048	3.195	3.422	1.10 ± 0.03
test/gtestp4c-nogc --gtest_filter=P4CParserUnroll.switch_20160512	2.948 ± 0.061	2.855	3.041	1.00

So, we're 10% faster here (!)

Runtime impact with GC on is much less visible due to huge GC overhead (1.5x):	Command	Mean [s]	Min [s]	Max [s]	Relative
test/gtestp4c-main --gtest_filter=P4CParserUnroll.switch_20160512	4.584 ± 0.099	4.312	4.897	1.02 ± 0.03
test/gtestp4c --gtest_filter=P4CParserUnroll.switch_20160512	4.512 ± 0.072	4.309	4.649	1.00

[asl]

sanitizers are broken after https://github.com/p4lang/p4c/pull/4989

[asl]

I rebased the branch after abseil string helpers were merged in. Now it should show the changes more cleanly

[asl]

validate-p4 fixes are in https://github.com/p4gauntlet/toz3/pull/41

[fruffy]

@smolkaj fyi. This might influence the way you work with annotations.

[vlstill]

@asl, apparently this breaks dbprint of annotated types, e.g. header of a function now can look like this in dbprint: { }void foo({ }in int<16> bar, { }in int<16> baz, { }out int<16> boo). Presumably the problem is that if annotations member is just printed, it will be printed using IR::Vector::dbprint, which is printed as bracket-encodes list with , separators.

[asl]

@vlstill The debug printing of annotations was pretty broken before this PR. In particular it only prints expr, so unparsed or structured annotations are not printed at all(!).

I'm working on the proper fix when porting Annotation to disjoint union. Stay tuned :)