Fix the position-check for ed25519 certs to work with annotations

Fix the position-check for ed25519 certs to work with annotations

When there are annotations on a router descriptor, the
ed25519-identity element won't be at position 0 or 1; it will be at
router+1 or router-1.

This patch also adds a missing smartlist function to search a list for
an item with a particular pointer.