Allow default values in hana define struct

Current usage looks like this:

```
struct Person {
BOOST_HANA_DEFINE_STRUCT(Person,
(std::string, name),
(int, age)
);
};
```
It's not clear from this whether the members will be declared as `int age;` or `int age{};`. Either way, one can't easily control the behavior. You can write a default constructor for Person by hand, but this involves repeating all the members and still has some limitations where multiple constructors are involved compared to inline initializers. Inline initializers are a fantastic feature and it would be fantastic to be able to simply write:

```
struct Person {
BOOST_HANA_DEFINE_STRUCT(Person,
(std::string, name, "Bob"),
(int, age, 42)
);
};
```

Or something similar (you could for example, make the user write anything they want to be appear between the variable name and the semi colon, so it would instead be `(int, age, =42)`).

This would be very useful for my purposes; I use Hana with POD structs but sometimes it would be nice to control their default construction without lots of boilerplate.

Start by locating the implementation and documentation for BOOST_HANA_DEFINE_STRUCT, then review the issue's discussion before choosing a syntax. Done means the macro supports the requested member default-value behavior and the existing behavior remains covered by the project's tests.

Written by the indexing model from the issue text.