multicursors: combine cursor and selection into one type

Waqar Ahmedrequested to merge
work/merge-cur-sel into master
  • Keeping them separate is making things painful needlessly and prevents us from sorting the cursors. This results in better overall performance and avoids a lot of needsless word.
  • fix crashes if we try to create a cursor in a line greater than lastLine or before 0
  • Rewrite selection merging so that it handles all cases correctly and add more tests for it

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