When updating to Go 1.23, consolidate ordmap and keylist into omap

[rcoreilly]

Describe the feature

newly-added keylist is simpler and avoids the KeyValue tuple from ordmap, but it is probably a bit slower on insert and delete, but almost all of our use-cases for this don't care about this.

Anyway, we won't revisit until we migrate to 1.23 and then we'll implement the iterators.

Relevant code

No response