low: proposal signal.union

[betula]

Collect signals from several values and signals. Worked as last value of set

const a = value(0);
const b = signal(0);

const u = signal.union(a, b);

u.val === 0;
a(5);
u.val === 5;
b(10);
u.val === 10;

[betula]

Added to "the stream of conciseness 0.7+ roadmap"