Support Anoma serialisation primitives as Juvix builtin axioms

[Paul Cadman]

The Anoma hoon stdlib now supports serialisation primitives:

• jam - serialization from a noun to an atom
• cue - deserialization from an atom to a noun, the inverse of jam.

We need to make these available as Juvix builtins, in the same way as we did for anomaGet.

Proposed builtins:

--- Encode a value
builtin anoma-encode
axiom anomaEncode : {A : Type} -> A -> Nat;

--- Decode a value
builtin anoma-decode
axiom anomaDecode : {A : Type} -> Nat -> A;