Add EtherCAT example to list slaves

[ulikoehler]

Stripped output:

$ cargo run --example list_slaves
Slave pos 0: SlaveInfo { name: "AS715N_sAxis_V0.10", ring_pos: 0, id: SlaveId { vendor_id: 4194304, product_code: 1813 }, rev: SlaveRev { revision_number: 12024, serial_number: 0 }, alias: 0, current_on_ebus: 0, al_state: PreOp, error_flag: 0, sync_count: 4, sdo_count: 0, ports: [SlavePortInfo { desc: MII, link: SlavePortLink { link_up: true, loop_closed: false, signal_detected: true }, receive_time: 979090354, next_slave: 65535, delay_to_next_dc: 0 }, SlavePortInfo { desc: MII, link: SlavePortLink { link_up: false, loop_closed: true, signal_detected: false }, receive_time: 1413563250, next_slave: 65535, delay_to_next_dc: 0 }, SlavePortInfo { desc: NotImplemented, link: SlavePortLink { link_up: false, loop_closed: true, signal_detected: false }, receive_time: 0, next_slave: 65535, delay_to_next_dc: 0 }, SlavePortInfo { desc: NotImplemented, link: SlavePortLink { link_up: false, loop_closed: true, signal_detected: false }, receive_time: 0, next_slave: 65535, delay_to_next_dc: 0 }] }
Discovered 1 slaves (checked positions 0..31)

The example compiles without any errors. I didn't want to add serde_json etc just for nicer formatting, since this is not really intended as a diagnostic tool, but more like a syntax example.

[flosse]

@ulikoehler
Thanks for your contribution!
Examples are great, they show how to use this crate and also act as some kind of "API test".
In principle, everything looks fine, just a few minor issues that still need to be resolved.

[ulikoehler]

@flosse Thanks for the quick review. I've already resolved all issues, and would appreciate feedback on the non-resolved issue regarding the comments.

About the CI pipeline failing, I believe this is unrelated to this PR, right?

[ulikoehler]

Thank you @flosse for the quick merge!