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)

Start at the for_each_field API and review the Boost.Hana-based multiple-visitation example in the issue, including the linked Compiler Explorer case. Determine how visitation across multiple aggregates should behave, then add coverage showing that the API invokes the callable for corresponding fields across the supplied objects.

Written by the indexing model from the issue text.