Remove default implementation for the DecCBOR class

[f-f]

Description

First part of #5513: we remove the default implementation for DecCBOR through FromCBOR, so that types that contain Bytestrings are more easily routed to the version-aware decoding

Checklist

• Commits in meaningful sequence and with useful messages.
• Tests added or updated when needed.
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• Versions updated in .cabal and CHANGELOG.md files when necessary, according to the
  versioning process.
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  NOTE: If bounds change in a cabal file, that package itself must have a version increase. (See RELEASING.md).
• Code formatted (use scripts/fourmolize.sh).
• Cabal files formatted (use scripts/cabal-format.sh).
• CDDL files are up to date (use scripts/gen-cddl.sh)
• hie.yaml updated (use scripts/gen-hie.sh).
• Self-reviewed the diff.

[lehins]

There is really no need for all of those extra top level definitions for reusability in both types of decoders, they only bring noise.
It is better to use special helper fromEraCBOR (see my suggestion)

We could even potentially add a newtype like this to Cardano.Ledger.Core.Era:

newtype EraDecoder era a = EraDecoder a

instance (Era era, DecCBOR a) => FromCBOR (EraDecoder era) where
  fromCBOR = fromEraCBOR @era 

which can then be used to do deriving via.

[lehins]

We actually need to create a DecCBOR instance for PackedBytes. For now that instance will simply biggy back on FromCBOR instance, but then we will need to export some functionality for safe construction of PackedBytes from this module: `https://github.com/IntersectMBO/cardano-base/blob/d78f2f350017f0b3a1c57f483a2c323676756622/cardano-crypto-class/src/Cardano/Crypto/PackedBytes.hs

Suggested change

	decCBOR = do
	sbs <- decCBOR @ShortByteString
	maybe (fail "Invalid hash size") pure $ hashFromBytesShort sbs
	{-# INLINE decCBOR #-}
	decCBOR = hashFromPackedBytes <$> decCBOR
	{-# INLINE decCBOR #-}

[f-f]

Ah I see - that's a hidden module so even the FromCBOR instance is not exported. I guess this is blocked on exposing at least that from base?