BOOST_HANA_ADAPT_STRUCT for templates?

I would love to be able to adapt structs like this:

```
template
struct foo {
T member;
}

BOOST_HANA_ADAPT_STRUCT_TPL(T, foo, member);
```

Looking at the `accessors_impl` interface, it feels like something doable...

Start by reading the accessors_impl interface and the existing BOOST_HANA_ADAPT_STRUCT adaptation behavior. Determine the required behavior for the templated foo example in the issue, including how its template parameter is handled. Done means templated structs can be adapted through the requested macro with coverage for the demonstrated case.

Written by the indexing model from the issue text.