openpgp: Wrap bare email addresses in angle brackets.

• Change UserID::from_address to wrap a bare address in angle brackets. That is, change UserID::from_address so that UserID::from_address(None, None, "alice@example.org") maps to <alice@example.org> not alice@example.org. This has the advantage that the standard regular expression for scoping CAs can match bare email addresses, see:

  https://docs.sequoia-pgp.org/sequoia_openpgp/regex/index.html#caveat-emptor

  • Fixes #1076 (closed).