`struct StableRef`: Refactor out from `CArc::{,base_}stable_ref` and … #867

Job	Run time
test-on-ubuntu-latest (x86_64-unknown-linux-gnu, release, --release --no-default-features --featu...	17m 20s
test-on-ubuntu-latest (x86_64-unknown-linux-gnu, release, --release)	9m 39s
	26m 59s

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