std: simplify host lookup
The logic for splitting up a string into a hostname and port is currently duplicated across (nearly) all of the networking implementations in `sys`. Since it does not actually rely on any system internals, this PR moves it to the `ToSocketAddr` implementation for `&str`, making it easier to discover and maintain.
On the other hand, the `ToSocketAddr` implementation (or rather the `resolve_socket_addr` function) contained logic to overwrite the port on the socket addresses returned by `LookupHost`, even though `LookupHost` is already aware of the port and sets the port already on Xous. This PR thus removes this logic by moving the responsibility of setting the port to the system-specific `LookupHost` implementation.
As a consequence of these changes, there remains only one way of creating `LookupHost`, hence I've removed the `TryFrom` implementations in favour of a `lookup_host` function, mirroring other, public iterator-based features.
And finally, I've simplified the parsing logic responsible for recognising IP addresses passed to `<(&str, u16)>::to_socket_addrs()` by using the `FromStr` impl of `IpAddr` rather than duplicating the parsing for both IP versions.
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