Improve memory reservation for `insert_entry` #427

Job	Run time
clippy	20s
miri	4m 45s
Check MSRV and minimal-versions	1m 52s
nostd_build (1.63.0, thumbv6m-none-eabi)	1m 39s
tests (1.63.0)	2m 12s
nostd_build (stable, thumbv6m-none-eabi)	17s
tests (stable, arbitrary)	44s
tests (stable, quickcheck)	41s
tests (stable, rayon)	48s
tests (stable, rustc-rayon)	45s
tests (stable, serde)	46s
tests (stable, borsh)	50s
tests (stable, std)	41s
tests (beta)	40s
tests (nightly, test build benchmarks)	36s
Success	1s
	17m 37s

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