Every field of Peer now has a corresponding unit struct defined in the
`peer_field` module, which implements the `PeerFieldDef` trait which in
turn contains information about the field's name, tarantool type and
rust type.

This information currently is used in the `field_by_raft_id` method
and is generated by the `define_peer_fields` macro, which also
automates the generation of the format (`peer_format` function) for the
"raft_group" space.