Fix quadratic runtime when updating region highlighter matches

Running %sYeti<ret>casdf on file
[example.journal.txt](https://github.com/mawww/kakoune/issues/4685#issuecomment-1193243588)
can cause noticeable lag.  This is because we insert text at 6000
selections, which means we need to update highlighters in those lines.
The runtime for updating range highlighters is quadratic in the
number of selections: for each selection, we call on_new_range(),
which calls add_matches(), which calls std::rotate(), which needs
needs linear time.

Fix the quadratic runtime by calling std::inplace_merge() once instead
of repeatedly calling std::rotate().  This is works because ranges
are already sorted.

I used this script to benchmark the improvements.
(In hindsight I could have just used "-ui json" instead of tmux).

	#!/bin/sh
	set -ex
	N=${1:-100}
	kak=${2:-./kak.opt}
	for i in $(seq "$N")
	do
		echo -n "\
	2022-02-06 * Earth
	    expense:electronics:audio    116.7 USD
	    liability:card              -116.7 USD

	2022-02-06 * Blue Yeti USB Microphone
	    expense:electronics:audio    116.7 USD
	    liability:card              -116.7 USD

	"
	done > big-journal.ledger
	echo > .empty-tmux.conf 'set -sg escape-time 5'
	test_tmux() {
		tmux -S .tmux-socket -f .empty-tmux.conf "$@"
	}
	test_tmux new-session -d "$kak" big-journal.ledger
	test_tmux send-keys '%sYeti' Enter c 1234567890
	sleep .2
	test_tmux send-keys Escape
	while ! test_tmux capture-pane -p | grep 123
	do
		sleep .1
	done
	test_tmux send-keys ':wq' Enter
	while test_tmux ls
	do
		sleep .1
	done
	rm -f .tmux-socket .empty-tmux.conf

This script's runtime used to grow super-linearly but now it grows
linearly:

	         kak.old  kak.new
	N=10000    1.142    0.897
	N=20000    2.879    1.400

Detailed results:

	$ hyperfine -w 1 './bench.sh 10000 ./kak.opt.'{old,new}
	Benchmark 1: ./bench.sh 10000 ./kak.opt.old
	  Time (mean ± σ):      1.142 s ±  0.072 s    [User: 0.252 s, System: 0.059 s]
	  Range (min … max):    1.060 s …  1.242 s    10 runs

	Benchmark 2: ./bench.sh 10000 ./kak.opt.new
	  Time (mean ± σ):     897.2 ms ±  19.3 ms    [User: 241.6 ms, System: 57.4 ms]
	  Range (min … max):   853.9 ms … 923.6 ms    10 runs

	Summary
	  './bench.sh 10000 ./kak.opt.new' ran
	    1.27 ± 0.09 times faster than './bench.sh 10000 ./kak.opt.old'
	$ hyperfine -w 1 './bench.sh 20000 ./kak.opt.'{old,new}
	Benchmark 1: ./bench.sh 20000 ./kak.opt.old
	  Time (mean ± σ):      2.879 s ±  0.065 s    [User: 0.553 s, System: 0.126 s]
	  Range (min … max):    2.768 s …  2.963 s    10 runs

	Benchmark 2: ./bench.sh 20000 ./kak.opt.new
	  Time (mean ± σ):      1.400 s ±  0.018 s    [User: 0.428 s, System: 0.083 s]
	  Range (min … max):    1.374 s …  1.429 s    10 runs

	Summary
	  './bench.sh 20000 ./kak.opt.new' ran
	    2.06 ± 0.05 times faster than '../repro.sh 20000 ./kak.opt.old'

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