[frost] Make "tweaks" mutate #803

Job	Run time
clippy_check	30s
Rustfmt	13s
build	1m 22s
doc-build	24s
test (x86_64-unknown-linux-gnu)	1m 12s
test-alloc (secp256kfun)	1m 6s
test (armv7-unknown-linux-gnueabihf)	7m 7s
test-minimal (secp256kfun)	1m 3s
test-alloc (sigma_fun)	55s
test-nightly	1m 31s
test-minimal (sigma_fun)	2m 48s
test-alloc (ecdsa_fun)	50s
test-minimal (ecdsa_fun)	43s
test-alloc (schnorr_fun)	52s
test-minimal (schnorr_fun)	57s
	21m 33s

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