Use mirai::call_mirai_() instead of manual polling #61

Job	Run time
windows-latest (devel)	5m 54s
windows-latest (release)	5m 53s
windows-latest (oldrel)	6m 30s
macOS-latest (devel)	3m 8s
macOS-latest (release)	3m 25s
macOS-latest (oldrel)	3m 32s
ubuntu-latest (devel)	2m 26s
ubuntu-latest (devel) mirai & nanonext devel	3m 20s
ubuntu-latest (release)	2m 53s
ubuntu-latest (oldrel)	2m 46s
ubuntu-latest (3.6)	2m 45s
	42m 32s

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