Consider using {} instead of () when constructing objects in samples

For example, in this:

```
struct id
{
template
T operator()(T x) const
{
return x;
}
};

auto int_result = boost::fit::result(id());
static_assert(std::is_same::value, "Not the same type");
```

Consider changing the second-to-last line to:

`auto int_result = boost::fit::result(id{});`

This makes it obvious that the use of "id" is a construction, not a call.