Cardano Updates

* `NonZero`
* `CompactForm Coin`
* `Credential`
* `KeyHash`
* `ScriptHash`
* and `StakeWithDelegation`

Fix `metadatum` decoder and `chain_code` CDDL

Test that bytes/text up to 64 bytes decode successfully, and that
values exceeding 64 bytes are rejected.

Changes made with the help of an LLM

The metadatum decoder now only enforces bytes/text size limits (0..64)
for decoder versions > 2, matching the previous SoftForks.validMetadata
behavior. Tests are added to roundTripShelleyCommonSpec so they run for
every era: accepting oversized values at version 2 (Shelley) and
rejecting them from version 3 (Allegra) onwards.

Changes made with the help of an LLM

The CDDL spec requires `bytes .size (0..64)` and `text .size (0..64)`
for transaction metadatum, but the decoder did not enforce these limits.
This moves the size validation into `decodeMetadatum` so it aligns with
the CDDL spec. The `Arbitrary Metadatum` instance is also constrained
to produce valid values, and the previously disabled CDDL anti-tests
are now enabled.

Changes made with the help of an LLM

The decoder now enforces bytes/text size limits (0..64) for metadatums,
making the rule-level validMetadatum check redundant. Remove
validMetadatum, the SoftForks.validMetadata gate, and the Allegra
InvalidMetadata imp test that relied on constructing invalid metadatums
in-memory.

Changes made with the help of an LLM