Make Word.MergeAll sub-quadratic #7872

The current implementation of Word.MergeAll uses Vec<T>.Remove() to remove symbols from the list:

Based on data I found, it seems it's common that around 75 % of symbols are removed here (though the data conflates multiple steps, so it could be not fully relevant). This makes the method quadratic, likely significantly affecting its performance.

We should consider improving that. (This won't make the method linear, since the use of PriorityQueue<T> leads to at least O(n log n).)