Support first class mixins #1729

Job	Run time
lint	16s
spec (alpine-latest, 3.0)	31s
spec (alpine-latest, 3.1)	43s
spec (alpine-latest, 3.2)	32s
spec (macos-latest, 3.0)	45s
spec (alpine-latest, 3.3-rc)	56s
spec (macos-latest, 3.1)	40s
spec (macos-latest, 3.2)	31s
spec (macos-latest, 3.3)	57s
spec (macos-latest, jruby)	2m 58s
spec (macos-latest, truffleruby)	2m 37s
spec (macos-latest, truffleruby+graalvm)	9m 36s
spec (ubuntu-latest, 3.0)	17s
spec (ubuntu-latest, 3.1)	21s
spec (ubuntu-latest, 3.2)	21s
spec (ubuntu-latest, 3.3)	27s
spec (ubuntu-latest, jruby)	1m 44s
spec (ubuntu-latest, truffleruby)	1m 28s
spec (ubuntu-latest, truffleruby+graalvm)	6m 21s
spec (windows-latest, 3.0)	2m 1s
spec (windows-latest, 3.1)	1m 26s
spec (windows-latest, 3.2)	1m 55s
spec (windows-latest, jruby)	4m 21s
release	0s
	41m 44s

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