Add FixedSizeBytes, lift VRF sizes to the type level

[Soupstraw]

Description

This PR lifts the sizes of VerKeyVRF, SignKeyVRF and CertVRF to the type level, which is similar to what we already do for DSIGN and KES. I also added the FixedSizeBytes typeclass, which gives a uniform interface to all the rawDeserialise* and encode* functions for DSIGN/KES/VRF types.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• All visible changes are prepended to the latest section of a CHANGELOG.md for the affected packages.
  New section is never added with the code changes. (See RELEASING.md)
• When applicable, versions are updated in .cabal and CHANGELOG.md files according to the
  versioning process.
• The version bounds in .cabal files for all affected packages are updated.
  If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• Self-reviewed the diff

[neilmayhew]

LGTM. My comments are just suggestions.

[lehins]

I am blocking this PR because I want to give my review before it gets merged

[lehins]

This is a very nice direction! Still needs a bit of love though to make ideal.

[lehins]

We need to reduce duplication if we introducing this new interface, which requires inverting the approach of rawEncodeFixedSized/rawDecodeFixedSized using rawSerialise*/rawDeserialise*

[lehins]

I think you missed the point of my suggestion about MonadFail

The goal is to:

• avoid duplicate logic of checking the size
• improve failure messages
• and later deprecation or extraction into standalone functions of individual rawDeserialise* functions.

Which means we need an instance of FixedSizeCodec for each algorithm, eg. instance FixedSizeCodec (VerKeyDSIGN Ed25519DSIGN) and all of the raw(Des|S)erialise* can get this implementation rawDeserialise* = rawDecodeFixedSized

Suggested change

	-- | Build a raw fixed-size decoder, failing with an informative message when
	-- the underlying decoder returns 'Nothing'. The first argument should be a
	-- noun describing the type (e.g. "key", "signature")
	mkRawFixedSizedDecoder ::
	forall a m.
	(Typeable a, KnownNat (FixedSize a), MonadFail m) =>
	String -> (BS.ByteString -> Maybe a) -> BS.ByteString -> m a
	mkRawFixedSizedDecoder name rawDecode bs =
	maybe (failFixedSized name (Proxy @a) bs) pure (rawDecode bs)
	{-# INLINE mkRawFixedSizedDecoder #-}