Refactor OsInputOutput (combine interfaces & frames into single Vec)

[cyqsimon]

Synopsis

OSInputOutput definition changed from:

pub struct OsInputOutput {
    pub network_interfaces: Vec<NetworkInterface>,
    pub network_frames: Vec<Box<dyn DataLinkReceiver>>,
    pub get_open_sockets: fn() -> OpenSockets,
    pub terminal_events: Box<dyn Iterator<Item = Event> + Send>,
    pub dns_client: Option<dns::Client>,
    pub write_to_stdout: Box<dyn FnMut(String) + Send>,
}

to:

pub struct OsInputOutput {
    pub interfaces_with_frames: Vec<(NetworkInterface, Box<dyn DataLinkReceiver>)>,
    pub get_open_sockets: fn() -> OpenSockets,
    pub terminal_events: Box<dyn Iterator<Item = Event> + Send>,
    pub dns_client: Option<dns::Client>,
    pub write_to_stdout: Box<dyn FnMut(String) + Send>,
}

This is to provide stronger semantic safeguards against accidental & unnoticed screwups.

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Unresolved issues

• This refactor seems to have changed test outputs substantively, when it really shouldn't have changed anything. This needs to be investigated before merge. Okay clearly not. That seems to be due to another commit on my local branch. This is good to merge then, seeing that the test failures are no worse than they were.