Fix reverse dns lookups on static binds

[PetrichorIT]

Turmoil supports the creation of clients/hosts bound to any address that impl ToIpAddr.
This allows for then creation of nodes, bound to static IP addresses. Such nodes
will not appear in the internal mappings of Dns, since they have no hostname.

Dns::reverse however expects that all nodes (all inputs would be more correct) are contained
in the dns mapping. If this assumption is broken, Dns::reverse panics. This means that any use of
Dns::reverse in combination with a statically bound address will panic.

The problem: If a tracing subscriber is active, internal traces will attempt to use Dns::reverse once
when the node is initialized, thus crashing the simulation (world.rs:107).

Solutions

I See three solutions:

A (currently implemented)

If a reverse lookup does not find any entry in the dns mapping, it returns the input address as a String.
This would keep API changes small (&str to String). However all input addresses that have not appeared
in the mapping would be handled this way, not just statically bound addresses.

B

Add static binds to dns mapping. We could implement node creation in a way, that a dns entry
mapping the stringified static address to the real static address is created. This would not
change the Dns::reverse, but major changes in address resolution / node creation would be nessecary.

C

Change the API of Dns::reverse to reflect the failable nature of such lookups. By changing the return type to either
Option<&str> or Result<&str> we could deal with failing lookups without panicking. This would be the most expressiv
option, but it would change the public API of Dns::reverse and Sim::reverse_lookup_pair.

[mcches]

Nice catch! Let me know what you think about the hybrid approach.

[mcches]

Lgtm! Thanks for all the recent improvements.