Add member type to `member_descriptor`

I have a use case where I would like to process types of members of a described struct. While it is possible to deduce the member type from `member_descriptor::pointer`, I think it would be helpful if the type was readily available as a `member_descriptor::type` typedef. This would save the user from having to define helper traits such as this:

```
template< typename T >
struct member_type;

template< typename T, typename U >
struct member_type< T (U::*) >
{
using type = T;
};

template< typename Descr >
struct deduce_member_type :
member_type< std::remove_cv_t< decltype(Descr::pointer) > >
{
};
```

Locate the definition of `member_descriptor` and the existing tests for member descriptors. Confirm how its `pointer` is exposed, then add coverage showing the requested member type is available and run the relevant test suite; done means the typedef is usable without a helper trait.

Written by the indexing model from the issue text.