FD-801 Misc. Identity Fixes

[sambarnes]

The following changes were made:

• (Entry Structure) In an identity chain's first entry, change element "identity-version" in JSON descriptor to "version". JSON decoders in some languages have trouble with the hyphen (for example they'd try to unmarshal it into a variable identity.identity-version which wouldn't be syntactically allowed)
• (Entry Structure) In key replacement entries, sign (<identity_chain_id> + <old_key> + <new_key>) to allow for people to reuse keys across identities (not recommended, but still want to close the possibility of a replay across identity chains)
• (Parsing Rule) In key replacements, add a rule such that a key can not appear more than once within the hierarchy