Clean the `agg()` implementation for `DataFrame` #350

Job	Run time
build (ubuntu-20.04, x86_64-unknown-linux-gnu)	7m 50s
build (ubuntu-20.04, aarch64-unknown-linux-gnu)	8m 50s
build (macos-latest, x86_64-apple-darwin)	25m 16s
build (macos-latest, aarch64-apple-darwin)	23m 37s
build (windows-latest, x86_64-pc-windows-gnu)	14m 49s
test (macos-latest, release)	4m 42s
test (windows-latest, oldrel-1)	6m 29s
test (windows-latest, release)	5m 10s
test (windows-latest, devel)	4m 23s
test (ubuntu-latest, oldrel-1)	2m 19s
test (ubuntu-latest, release)	1m 58s
test (ubuntu-latest, devel)	2m 22s
release	0s
	1h 47m 45s

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