Split edge_connection_t into edge_connection_t and entry_connection_t
See branch "split_entry_conn" in my public repository.
It divides off the entry-only fields from edge_connection_t, and makes a new type for them.
This isn't a material memory savings: we get less than 100 bytes saved per exit connection by moving the fields out of edge_connection_t. Rather, it's meant to make the code more intelligible by making edge_connection_t contain only the common edge connection fields.