Switch Tabled trait from Vec to [_; Self::LENGTH]

[zhiburt]

Just short idea came to me that we could switch to a constant vector.
And everything will be fine.

The only question I've not investigated is.
Is there ANY reason why we shall stick with Vec?

tabled/src/tabled.rs

Lines 10 to 21 in 27ba1ca

	pub trait Tabled {
	/// A length of fields and headers,
	/// which must be the same.
	const LENGTH: usize;
	/// Fields method must return a list of cells.
	///
	/// The cells will be placed in the same row, preserving the order.
	fn fields(&self) -> Vec<Cow<'_, str>>;
	/// Headers must return a list of column names.
	fn headers() -> Vec<Cow<'static, str>>;
	}

[zhiburt]

Seems fine to do so.

[zhiburt]

Yet not possible cause rust-lang/rust#60551