R-CMD-check

Replace Rcpp dependency with cpp11 #349

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Job	Run time
macos-latest (release)	1m 31s
windows-latest (release)	2m 34s
windows-latest (4.1)	4m 34s
ubuntu-latest (devel)	4m 13s
ubuntu-latest (release)	1m 45s
ubuntu-latest (oldrel-1)	2m 8s
ubuntu-latest (oldrel-2)	2m 24s
ubuntu-latest (oldrel-3)	1m 57s
ubuntu-latest (oldrel-4)	2m 19s
	23m 25s