Simplify variant protocol #1921

Job	Run time
msrv	1m 22s
docs	3m 54s
clippy	2m 22s
nightly	4m 49s
rustfmt	8s
basics	1s
test_lib	5m 10s
build_feature (alloc)	3m 38s
no_std (ubuntu-latest, minimal)	1m 18s
no_std (macos-latest, minimal)	1m 50s
test_doc	5m 38s
build_feature (cli)	4m 44s
test_rune	3m 33s
build_feature (cli,doc)	4m 55s
test_miri (rune)	3m 44s
wasm	1m 33s
build_feature (cli,fmt)	4m 29s
test_miri (rune-core)	1m 2s
build_feature (doc)	4m 21s
test_miri (rune-alloc)	6m 20s
build_feature (workspace)	3m 51s
build_feature (languageserver)	4m 24s
build_feature (byte-code)	3m 39s
build_feature (capture-io)	3m 50s
build_feature (emit)	3m 48s
	1h 24m 23s

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