Variants are not supported by parsers that lazily synthesize attributes e.g. `string`

If parser lazily synthesize attribute (e.g. `string`) it actually exposes a pair of iterators that will be used to build an attribute inplace by `qi::assign_to`/`x3::move_to` call.

But neither `x3::move_to(first, last, attr)` nor `qi::assign_to(first, last, attr)` support variants. So next code fails to compile while logically valid:

```cpp
#include
#include
#include
#include

int main()
{
typedef boost::variant attribute_type;
#if X3
{
namespace x3 = boost::spirit::x3;
x3::rule r;
auto start = r = x3::string("foo");

// need to have a parse call to instantiate the templates
attribute_type a;
std::string s = "foo";
x3::parse(s.cbegin(), s.cend(), start, a);
}
#else
{
namespace qi = boost::spirit::qi;
qi::rule r;
r = qi::string("foo");
}
#endif
return 0;
}
```

Start by compiling the reproducer in the issue with both the X3 and Qi paths enabled. Read the implementations of x3::move_to and qi::assign_to, focusing on how they handle iterator pairs and boost::variant attributes. Done means the shown examples compile successfully for both parser implementations.

Written by the indexing model from the issue text.