feat: switch from extendr to the savvy framework #91

Job	Run time
build (ubuntu-latest, x86_64-unknown-linux-musl)	33s
build (ubuntu-latest, aarch64-unknown-linux-musl)	50s
build (macos-latest, x86_64-apple-darwin)	21s
build (macos-latest, aarch64-apple-darwin)	36s
build (windows-latest, x86_64-pc-windows-gnu)	1m 32s
test (macos-latest, release)	2m 25s
test (windows-latest, oldrel-1)	5m 0s
test (windows-latest, release)	3m 10s
test (windows-latest, devel)	5m 9s
test (ubuntu-latest, oldrel-1)	2m 2s
test (ubuntu-latest, release)	1m 30s
test (ubuntu-latest, devel)	4m 55s
release	0s
	28m 3s

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