The tor_cert_get_checkable_sig function uses the signing key included in
the certificate (if available) when a separate public key is not given.
When the signature is valid, the tor_cert_checksig function copies the
public key from the checkable structure to the public key field of the
certificate signing key.
In situations where the separate public key is not given but the
certificate includes a signing key, the source and destination pointers
in the copy operation are equal and invoke undefined behavior.
Undefined behaviour is avoided by ensuring both pointers are different.
Make sure that signing certs are signed by the right identity key,
to prevent a recurrence of #16530. Also make sure that the master
identity key we find on disk matches the one we have in RAM, if we
have one.
This is for #16581.
For prop220, we have a new ed25519 certificate type. This patch
implements the code to create, parse, and validate those, along with
code for routers to maintain their own sets of certificates and
keys. (Some parts of master identity key encryption are done, but
the implementation of that isn't finished)
