Unify usage of 'issuer' and 'signer' as nouns

[aarongable]

We define a "signer" to be a private key, or something that satisfies the
crypto.Signer interface. We define an "issuer" to be an object which has
both a signer (so it can sign things) and a certificate (so that the things
it signs can have appropriate issuer fields set).

As a result, this change:

• moves the new "signer" library to be called "issuance" instead
• renames several "signers" to instead be "issuers", as defined above
• renames several "issuers" to instead be "certs", to reduce confusion more

There are some further cleanups which could be made, but most of them
will be made irrelevant by the removal of the CFSSL code, so I'm leaving
them be for now.

[jsha]

Sweet!