sieve: Use `saturating_add` when computing `next_base` #26

Job	Run time
tests (1.36.0)	4m 30s
tests (stable)	1m 19s
tests (beta)	1m 22s
tests (nightly)	1m 25s
tests (nightly-i686-unknown-linux-gnu)	2m 25s
tests (stable, s390x-unknown-linux-gnu)	3m 49s
bors	1s
	14m 51s

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