Consider adding dereference operator for signals

[YarikTH]

Description

It would be more convenient if to have a sum of two signals we will have

var_signal<int> a = make_var(1);
var_signal<int> b = make_var(1);

int c = *a + *b;

instead of

int c = a.get() + b.get();
//or
int d = a() + b();

It matches existing pointer semantic much more. Also assignment can be considered.

*a = 10;

To do so, some kind of proxy is required, because we can't just return T const& and T&.

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Hmm... as a silly though. If we add NOP operator++, then signals can be passed in std algorithms both as source and as a destination.

std::copy(vec.begin(), vec.end(), a);
std::copy(a.begin(), a.end(), std::back_inserter(vec));

Except the MSVC that requires iterators having the type traits(