Use explicit unused bytes instead of inferred for most padding

[Antikyth]

I think as a general code style rule, it is clearer to write:

#[derive(Debug, Hash, PartialEq, Eq, X11Size, Readable, Writable)]
pub struct SetFontPath: Request(51, error::Value) {
    #[allow(clippy::cast_possible_truncation)]
    let path_len: u16 = path => path.len() as u16,
    [_; 2],

    #[context(path_len => usize::from(*path_len))]
    pub path: Vec<LengthString8>,
    [_; path => pad(path)],
}

rather than:

#[derive(Debug, Hash, PartialEq, Eq, X11Size, Readable, Writable)]
pub struct SetFontPath: Request(51, error::Value) {
    #[allow(clippy::cast_possible_truncation)]
    let path_len: u16 = path => path.len() as u16,
    [_; 2],

    #[context(path_len => usize::from(*path_len))]
    pub path: Vec<LengthString8>,
    [_; ..],
}

I asked for some feedback for this from friends, and the first was strongly preferred for clarity and readability.

Unresolved questions

Should unused bytes for filling up to the minimum message limits be explicit, or stay as inferred? I'm leaning towards inferred, but feel free to discuss.

Inferred

pub struct GetSelectionOwner: Reply for request::GetSelectionOwner {
	#[sequence]
	#[derivative(Hash = "ignore", PartialEq = "ignore")]
	pub sequence: u16,

	pub owner: Option<Window>,
	[_; ..],
}

Explicit

pub struct GetSelectionOwner: Reply for request::GetSelectionOwner {
	#[sequence]
	#[derivative(Hash = "ignore", PartialEq = "ignore")]
	pub sequence: u16,

	pub owner: Option<Window>,
	[_; 20],
}