perf: avoid cloning precompiles #3666

Job	Run time
check no-default-features	21s
test stable --no-default-features	54s
test no_std	27s
test stable	54s
test stable --all-features	1m 30s
check serde	18s
test beta --no-default-features	1m 2s
test beta	55s
test beta --all-features	1m 28s
check std	24s
test nightly --no-default-features	2m 10s
clippy	1m 25s
test nightly	2m 16s
docs	1m 13s
test nightly --all-features	2m 27s
fmt	14s
	17m 58s

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