Migrate from reactive-box syntax to more oop

[betula]

Remove basic API functions:

• expr
• sel
• box

New API functions:

const a = value(0);
a.val += 10;
console.log(a.val);
a(11) // change value to 11

const s = selector(() => a.val + 10);
console.log(s.val);

Remove action

New API signal

const i = signal(10);
console.log(i.val);
i(10) // call signal

[betula]

#29