Sparse Array for Group By

This is just a thought, but we should consider making a sparse array similar to `hana::detail::hash_table` for the purpose of making a fast "group by" algorithm. One problem would be that it would have to use `fold_left` to construct the lookup which I recall was an expensive up front cost.

The result could even be a `hana::map` as it appears that everything except `make_map` is agnostic to the use of `hana::hash`.

I kind of got a start on something. The following is ripped from `hana::detail::hash`:

```cpp
namespace sparse_array
{
template
struct group { };

template
struct lookup_table
: Groups...
{ };

// find indices
template
std::index_sequence find_indices_impl(group const&);

template
std::index_sequence<> find_indices_impl(...);

template
struct find_indices {
using type = decltype(find_indices_impl(Lookup{}));
};

// update_group
template
struct update_group {
using type = Group;
};

template
struct update_group, Key, Index> {
using type = group;
};

// group_insert
template ::type::size() > 0)
>
struct group_insert;

template
struct group_insert, Node, Index, true> {
// There is a group for that Key; append the new index to it.
using type = lookup_table<
typename update_group::type...
>;
};

template
struct group_insert, Node, Index, false> {
// There is no group for that Key; insert a new group.
using type = lookup_table>;
};

template
struct make {
using type = typename decltype(
hana::fold_left(
std::declval(),
hana::type_c>,
hana::template_
)
)::type;
};
}
```

See the following for background information:
http://stackoverflow.com/questions/42541148/how-do-i-speed-up-many-different-hanafilters-on-the-same-input-data

No target file or test is identified. Start by reviewing hana::detail::hash, fold_left, and the linked background discussion to assess the proposed sparse lookup for group-by operations. The issue does not define an API, implementation location, or completion criteria, so those would need agreement before work can begin.

Written by the indexing model from the issue text.