Make key generation pure

[perturbing]

Currently. the usage of

cardano-base/cardano-crypto-class/src/Cardano/Crypto/Seed.hs

Line 82 in 4b27937

getBytesFromSeedT :: Word -> Seed -> (ByteString, Seed)

In some DSIGN instances makes the key generation not pure. Also, we know the Seed and key size at the type level, so we should use that. See this comment.