[frost] EncodedFrostKey BorrowDecode and correct markers #739

Job	Run time
build	25s
test (x86_64-unknown-linux-gnu)	53s
test (armv7-unknown-linux-gnueabihf)	5m 31s
clippy_check	15s
doc-build	15s
Rustfmt	14s
test-alloc (secp256kfun)	47s
test-minimal (secp256kfun)	49s
test-nightly	1m 27s
test-alloc (sigma_fun)	13s
test-minimal (sigma_fun)	13s
test-alloc (ecdsa_fun)	37s
test-minimal (ecdsa_fun)	45s
test-minimal (schnorr_fun)	40s
test-alloc (schnorr_fun)	47s
	13m 51s

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