[frost] Make SignSession serde #728

Job	Run time
build	3m 17s
clippy_check	27s
doc-build	7s
Rustfmt	14s
test (x86_64-unknown-linux-gnu)	1m 34s
test-alloc (secp256kfun)	1m 2s
test-nightly	1m 29s
test (armv7-unknown-linux-gnueabihf)	5m 51s
test-alloc (sigma_fun)	29s
test-minimal (secp256kfun)	1m 10s
test-alloc (ecdsa_fun)	44s
test-minimal (sigma_fun)	32s
test-minimal (ecdsa_fun)	43s
test-alloc (schnorr_fun)	59s
test-minimal (schnorr_fun)	52s
	19m 30s

Provide feedback