Add comparison operators for variant_object

[sim31]

Useful for eosio contract unit tests, when comparing expected rows and actual. Especially when compared objects have nested objects inside them.

[b1bart]

@sim31 thank you for submitting this, sorry it took so long to review.

In addition to addressing the feedback, this new feature should have related unit tests before we can accept it.

[b1bart]

the order which these iterators traverse a (mutable_)?variant_object is not strictly defined as ordered. As a result this comparison will return false when comparing these two objects:

auto lhs = mutable_variant_object()("foo", 1)("bar", 1);
auto rhs = mutable_variant_object()("bar", 1)("foo", 1);

// expect are_equal to be true but it is not!
bool are_equal = (lhs == rhs);