[frost] Make "tweaks" mutate #806

Job	Run time
build	1m 4s
clippy_check	28s
doc-build	18s
Rustfmt	11s
test (x86_64-unknown-linux-gnu)	1m 22s
test-alloc (secp256kfun)	55s
test (armv7-unknown-linux-gnueabihf)	7m 2s
test-minimal (secp256kfun)	47s
test-alloc (sigma_fun)	22s
test-alloc (ecdsa_fun)	2m 0s
test-nightly	1m 34s
test-minimal (sigma_fun)	22s
test-alloc (schnorr_fun)	49s
test-minimal (ecdsa_fun)	45s
test-minimal (schnorr_fun)	39s
	18m 38s

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