Add Protocol::SUB/SUB_ASSIGN to Instant #1881

Job	Run time
rustfmt	9s
msrv	1m 2s
docs	2m 57s
clippy	1m 49s
nightly	3m 1s
basics	1s
no_std (ubuntu-latest, minimal)	1m 5s
test_miri (rune)	3m 55s
test_miri (rune-core)	48s
no_std (macos-latest, minimal)	1m 5s
test_miri (rune-alloc)	6m 14s
test_rune	2m 7s
test_lib	3m 13s
build_feature (alloc)	2m 18s
test_doc	5m 39s
build_feature (cli)	3m 16s
wasm	54s
build_feature (cli,doc)	3m 10s
build_feature (cli,fmt)	3m 10s
build_feature (doc)	2m 44s
build_feature (workspace)	2m 30s
build_feature (languageserver)	3m 1s
build_feature (byte-code)	2m 14s
build_feature (capture-io)	2m 31s
build_feature (emit)	2m 37s
	1h 1m 30s

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