Move to structured logging

[jmezo]

Description

Changes logging statements from using string formatting to structured logging. Closes #222

Change checklist

• Self-review
• [ ] Documentation updates if relevant
• Tests if relevant

[ackintosh]

Thank you @jmezo, for this PR! I have left some comments.

Additionally, it would be appreciated if you could update the logging statements to add spaces around =, similar to the following:

discv5/src/handler/mod.rs

Lines 656 to 661 in ec561f2

	warn!(
	source = %request_call.contact(),
	message_nonce = hex::encode(request_call.packet().message_nonce()),
	expected_nonce = hex::encode(request_nonce),
	"Received a WHOAREYOU from a non expected source."
	);

[jmezo]

Thanks for the review and suggestions @ackintosh. I added the spaces around = in 4ca08a2.

Let me know if the commits should be squashed, or if there are any other issues.

[ackintosh]

FYI: The clippy errors will be fixed by #259

[ackintosh]

Looks good to me.