Add functions to convert from system keys

[chrisduerr]

Keycodes are almost never constructed by hand, but rather received from some third-party system library which provides raw keycodes/keysyms. Applications like winit are then forced to remap them to some intermediate type like the one provided in this crate.

To avoid having to write this conversion from system format to keyboard-types in every single application, it would make sense to centralize this in the keyboard-types crate. This would also likely benefit keyboard support overall, since there are some pretty niche keys that are usually rarely supported unless a person happens to be using them.

[madsmtm]

Would probably have to look something like the following IIUC:

impl Code {
    pub fn from_carbon_scancode(scancode: u32) -> Option<Self> { ... }
    pub fn to_carbon_scancode(&self) -> u32 { ... }
    pub fn from_xkb_keycode(keycode: u32) -> Option<Self> { ... }
    pub fn to_xkb_keycode(&self) -> u32 { ... }
    // ...
}

Because I'm not sure that we can reliably make a platform choice here just based on #[cfg(...)]? Depends on how e.g. X11 works on macOS (and Android), does it use XKB or something else?