Add `serde_ref` and `serde_ref_mut` utility modules #2253

Job	Run time
maybe-expedite	5s
lint	4m 16s
test (third-party, bincode, stable)	13m 22s
test (third-party, bincode, nightly)	10m 31s
test (third-party, postcard, stable)	11m 46s
test (third-party, postcard, nightly)	10m 35s
test (other, bincode, stable)	26m 30s
test (other, bincode, nightly)	31m 38s
test (other, postcard, stable)	26m 16s
test (other, postcard, nightly)	32m 17s
all-checks	1s
	2h 47m 17s

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