Add `serde_ref` and `serde_ref_mut` utility modules #2260

Job	Run time
maybe-expedite	5s
lint	4m 6s
test (third-party, bincode, stable)	11m 53s
test (third-party, bincode, nightly)	12m 0s
test (third-party, postcard, stable)	13m 14s
test (third-party, postcard, nightly)	11m 44s
test (other, bincode, stable)	24m 21s
test (other, bincode, nightly)	30m 19s
test (other, postcard, stable)	25m 26s
test (other, postcard, nightly)	31m 23s
all-checks	1s
	2h 44m 32s

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