Use `clone_from()` in two places #796

Job	Run time
clippy (beta)	27s
tests (stable)	4m 7s
cross_test (stable, powerpc-unknown-linux-gnu)	31m 21s
tests (beta)	5m 0s
cross_test (stable, i686-unknown-linux-gnu)	3m 21s
tests (nightly)	4m 29s
tests (1.51.0)	9m 32s
	58m 17s

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