[P4Testgen] Replace absl::btree_map with the P4::flat_map implementation

[fruffy]

The flat map implementation was donated by @asl. It seems a little faster than btree_map.

[asl]

@fruffy I think we can jus grab existing ordered_map tests?

[asl]

Added few tests and did some fixes here and there

[vlstill]

Why is a sorted map better here than e.g. absl::flat_hash_map? Do we need it to be sorted?

[asl]

Why is a sorted map better here than e.g. absl::flat_hash_map? Do we need it to be sorted?

IIRC the order did matter here. And there are lots of small maps here and there, so we need to reduce the overhead as well.

[fruffy]

Why is a sorted map better here than e.g. absl::flat_hash_map? Do we need it to be sorted?

We do not have a hashing implementation for symbolic variables either. It is added here but not as efficient as it could be. #4667

[vlstill]

We do not have a hashing implementation for symbolic variables either. It is added here but not as efficient as it could be. #4667

Yep, we don't need to squeeze all the last percents of performance in all case, I am fine with this using P4::flat_map, if in future it turns out absl::flat_hash_map would perform even better, we can always change it.