Add FrontendRegion, changing Frontend and PrettyScreen traits

[e-matteson]

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[justinpombrio]

struct FrontEndScreen<'a, T: FrontEnd> {
    frontend: &'a mut T,
    region: Region
}

trait Frontend {
    fn print();
    fn get_screen<'a>(region: &'a mut Region) -> FrontendScreen<'a, Self>;
}

impl<'a, T> PrettyScreen for FrontEndScreen<'a, T> {
    fn print(&mut self, pos) {
        self.frontend.print(OFFSET(self.region, pos));
    }
}

[justinpombrio]

(Should probably call it a FrontendScreen rather than a FrontendRegion, since the point is that it implements PrettyScreen.)

[e-matteson]

Closed by #19, though with different design and names.