chore: add defrag for StringSet

[kostasrim]

• add defrag logic for StringSet
• add test

Resolves #3822

[kostasrim]

Would maybe worth to add a unit with ttl to be 100% sure we copy it properly. So two tests in total:

1. As is now, without ttl
2. With ttl

[kostasrim]

Once I am back from the holidays I will see how to refactor those under one single function for all DenseSet variants. Furthermore, same applies for the tests

[romange]

It can wait for your return, I prefer not to merge it now

[chakaz]

Generally LG, but I'd love @romange to at least take a look at the ReallocIfNeeded part. Specifically is it always safe and true to do:

  auto* ptr = curr_entry_;
  if (ptr->IsLink()) {
    ptr = ptr->AsLink();
  }

  DCHECK(!ptr->IsEmpty());
  DCHECK(ptr->IsObject());

?

[kostasrim]

Generally LG, but I'd love @romange to at least take a look at the ReallocIfNeeded part. Specifically is it always safe and true to do:

  auto* ptr = curr_entry_;
  if (ptr->IsLink()) {
    ptr = ptr->AsLink();
  }

  DCHECK(!ptr->IsEmpty());
  DCHECK(ptr->IsObject());

?

Yep this is fine, links must be converted via AsLink to properly use. We do this in other places well, see for example: StringMap::ReallocIfNeeded.

In fact, we can unify some of those functions under the same interface -- something I wanted to do but I do not currently have the bandwidth for the small refactor