Cardano Updates

and add lower bound in core

to conform to the implementation.
Enable multiasset roundtrip tests.

and add lower bound in core

To get there we need to move the `ApplyTxError era` type to be a
data family in the ApplyTx class

This commit adds a mocked-up certificate validation function
validatePerasCert, as well as an empty PerasKey type encoding the
cryptographic key needed to validate Peras certificates when they
arrive in blocks.

Both of these types will be replaced with their corresponding ones
from 'cardano-base' once these are ready.