Feature request: for_each_field multiple visitation

Multiple visitation is easy enough to achieve with a library like Boost.Hana (see below), but I think it would be useful functionality for ```for_each_field``` to support directly.

```cpp
template
constexpr auto for_each_field(Invocable f, Aggregates&&... xs) -> void
{
namespace hana = boost::hana;
hana::for_each(hana::zip(boost::pfr::structure_tie(xs)...), hana::fuse(std::move(f)));
}
```
[Example on Compiler Explorer](https://godbolt.org/z/K9Krfh)