v: improved comptime var checking with is operator and smartcasting

[felipensp]

Fix #20312

type TestSum = string | int

fn gen[T, R](sum T) R {
	$if T is $sumtype {
		$for v in sum.variants {
			if sum is v {
				dump(sum)
				$if sum is R {
					return sum
				}
			}
		}
	}
	return R{}
}

fn main() {
	dump(gen[TestSum, string](TestSum('foo')))
}

[spytheman]

Excellent work.