Generalized compound assignment

[DerpMcDerp]

It's possible to get rid of the zoo of compound assignment operators and replace it with a general mechanism:

Generalized compound assignment

foo.x = $old + 1;
// foo.x += 1;

Generalized post-increment/decrement

old foo.x = $old + 1;
// foo.x++;

old foo.x = 3;
// std::exchange(foo.x, 3);