[frost] EncodedFrostKey BorrowDecode and correct markers #738

Job	Run time
build	19s
Rustfmt	11s
doc-build	15s
clippy_check	16s
test (x86_64-unknown-linux-gnu)	53s
test-alloc (secp256kfun)	48s
test (armv7-unknown-linux-gnueabihf)	5m 43s
test-minimal (secp256kfun)	50s
test-alloc (sigma_fun)	20s
test-nightly	29s
test-minimal (sigma_fun)	16s
test-alloc (ecdsa_fun)	41s
test-minimal (ecdsa_fun)	49s
test-alloc (schnorr_fun)	48s
test-minimal (schnorr_fun)	40s
	13m 18s

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