all: drop x/exp direct dependency

[holiman]

This is a not-particularly-important "cleanliness" PR. It removes the last remnants of the x/exp package, where we used the maps.Keys function.

The original returned the keys in a slice, but when it became 'native' the signature changed to return an iterator, so the new idiom is slices.Collect(maps.Keys(theMap)), unless of course the raw iterator can be used instead.

In some cases, where we previously collect into slice and then sort, we can now instead do slices.SortXX on the iterator instead, making the code a bit more concise.

This PR might be slighly less optimal, because the original x/exp implementation allocated the slice at the correct size off the bat, which I suppose the new code won't.

Putting it up for discussion.

[holiman]

Ah, the slices.SortedFunc requires 1.23. Guess this PR will have to wait