Tests

xtask: added a `cargo extendr document` #122

Job	Run time
ubuntu-latest w/ bindgen (R-devel rust-stable )	4m 12s
windows-latest (R-release rust-stable-msvc)	10m 14s
macOS-latest w/ bindgen (R-devel rust-stable )	13m 45s
windows-latest (R-devel rust-stable-msvc)	10m 53s
windows-latest w/ bindgen (R-release rust-stable-msvc x86_64-pc-windows-gnu)	6m 43s
windows-latest (R-oldrel rust-stable-msvc)	14m 42s
windows-latest w/ bindgen (R-devel rust-stable-msvc x86_64-pc-windows-gnu)	6m 12s
macOS-latest (R-release rust-stable)	12m 13s
windows-latest w/ bindgen (R-release rust-nightly-msvc x86_64-pc-windows-gnu)	8m 34s
ubuntu-latest (R-release rust-stable)	53s
windows-latest w/ bindgen (R-oldrel rust-nightly-msvc x86_64-pc-windows-gnu)	11m 5s
ubuntu-latest (R-release rust-nightly)	5m 54s
ubuntu-latest (R-devel rust-stable)	5m 2s
ubuntu-latest (R-oldrel rust-stable)	6m 14s
	1h 56m 36s

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