Use println! macros in default host writer function to be able to capture output during tests

[ludfjig]

When a guests prints, the host will print to terminal in green color using the termcolor crate. Since we don't use the println! macros for this, the stdout output is not captured by tests. I personally don't think the green color is enough to justify the different behavior from println! style macros, so I think we should switch it out. If we want, we could have a prefix instead, maybe something like [guest] hello world!.