Refactor Descr.map_all_but_one

[sabiwara]

Another mechanical refactor.

Comparing map keys by casting them into sets is much slower and memory intensive than checking map size + loop over the keys (bench).

Also using Enum.count_until to bail at the second different key found.

We could probably avoid the extra Enum.find pass by re-using the pattern from map fusion, but we can't reuse the exact same code because we're not checking for subtypes here (well, perhaps we could actually leverage this information and do something smart?)

Finally, removed the ? from the name since it doesn't return a bool.