Add back Console::drop

[AzureMarker]

This is needed for soundness. Otherwise libctru will have a dangling pointer and will write to it when printing.

Closes #15 (I think, maybe we also need some panic changes before closing?)

[Meziu]

Testing your branch against a simple "make console, drop it and then print" binary, made it crash on drawChar:

Does it work to you?

[Meziu]

Just to be on the same page, my main is:

    ctru::init();
    let gfx = Gfx::default();
    let hid = Hid::init().expect("Couldn't obtain HID controller");
    let apt = Apt::init().expect("Couldn't obtain APT controller");
    let _console = Console::default();
    drop(_console);

    println!("Works");

[AzureMarker]

I guess I had the misconception that the initial selected console value is the default console. It's actually a zeroed-out console:
https://github.com/devkitPro/libctru/blob/5f13628dac75206f0c97d29a7427ce8284d910f1/libctru/source/console.c#L65-L67

(I was wondering if it would use uninitialized memory or zeroed memory, and found this: https://stackoverflow.com/questions/4080673/are-the-members-of-a-global-structure-initialized-to-zero-by-default-in-c)

So I'll change it to set the console to some zeroed out console on drop, if it was the selected console.

[AzureMarker]

Unfortunately right now std::mem::zeroed is not const:
rust-lang/rust#62061

So I used the recommended workaround:
https://crates.io/crates/const-zero

[AzureMarker]

It's no longer simpler, but at least it's safe.

[Meziu]

Nice, not printing once the Console is dropped is both expected behaviour and easy to use.