Support "otherwise" in split

[ice1000]

Expected syntax:

let threee: U = sum { One | Two | Three };
let three: U = sum { Two | Three };

let three_f (a: U) (m: three): 1 = split { Two => 0 | Three => 0 };
let threee_f (a: U) (m: threee): 1 = split { One => 0 | e => (three_f a) e };