Support first class mixins #1723

Job	Run time
lint	14s
spec (alpine-latest, 3.0)	36s
spec (alpine-latest, 3.1)	37s
spec (macos-latest, 3.0)	49s
spec (alpine-latest, 3.2)	29s
spec (macos-latest, 3.1)	26s
spec (alpine-latest, 3.3-rc)	52s
spec (macos-latest, 3.2)	35s
spec (macos-latest, 3.3)	49s
spec (macos-latest, jruby)	2m 38s
spec (macos-latest, truffleruby)	3m 7s
spec (macos-latest, truffleruby+graalvm)	9m 18s
spec (ubuntu-latest, 3.0)	23s
spec (ubuntu-latest, 3.1)	25s
spec (ubuntu-latest, 3.2)	19s
spec (ubuntu-latest, 3.3)	32s
spec (ubuntu-latest, jruby)	1m 41s
spec (ubuntu-latest, truffleruby)	1m 34s
spec (ubuntu-latest, truffleruby+graalvm)	6m 8s
spec (windows-latest, 3.0)	1m 38s
spec (windows-latest, 3.1)	1m 48s
spec (windows-latest, 3.2)	3m 23s
spec (windows-latest, jruby)	2m 53s
release	0s
	41m 14s

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