v: allow sumtype init by variant comptime var T(v) / SumType(v)

[felipensp]

This PR allows to initialize Sumtype through its variant comptime types.

e.g.

type Sum = int | string

fn get[T](val T, type_name string) T {
	$if T is $sumtype {
		$for v in val.variants {
			if type_name == typeof(v.typ).name {
				return T(v)
			}
		}
	}
	return T{}
}

fn main() {
	dump(get(Sum{}, 'int')) // Sum(0)
	dump(get(Sum{}, 'string')) // Sum('')
}

_{Huly®: V_0.6-21113}

[spytheman]

Excellent work.