Repeat arguments

[Tamschi]

(This waits on me completing idanarye/rust-typed-builder#46.)

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This feature allows adding * and + to constructor and render argument definitions so that

asteracea::component! {
	Any(
		pub sometimes*?: usize,
		pub some/many+?: usize,
		pub one/any*: usize,
		pub always+: usize,
	)()

	new with {
		let _: &Option<Vec<usize>> = &sometimes;
		let _: &Option<Vec1<usize>> = &many;
		let _: &Vec<usize> = &any;
		let _: &Vec1<usize> = &always;
	}

	[]
}

is legal.

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Open Questions

• Should *? imply Option<Vec<…>> or just Vec<…>?