Try to support transparent usage of operators

[YarikTH]

I would like to make the following code possible:

ureact::var_signal<int> a = make_var(&c, 1);
ureact::var_signal<int> b = make_var(&c, 1);
if( a < b ){
}else if( a > b ){
}else if( a == b ){
}

a = a + b;
a += b;

The main difference with the current usage is the absence of the .value() method calls.
a = b + c is much clear than a.set_value( b.value() + c.value() )

But now operators are overloaded to make easy signals creation. So need to investigate what can be done with it.

[YarikTH]

Not needed