core: simplify compute_refreshing_shares() #1719

Job	Run time
test on beta	2m 28s
Clippy	45s
build with latest versions of dependencies	44s
build with default features	29s
Check if automatically generated code is up to date	13s
build with MSRV (1.81)	51s
Rustfmt	8s
build with no_std (ristretto255)	22s
Check Rust doc	30s
actionlint	13s
build with no_std (secp256k1)	26s
build with no_std (ed25519)	38s
build with no_std (secp256k1-tr)	35s
build with no_std (p256)	19s
build with no_std (rerandomized)	20s
	9m 1s

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