Hashable
Concepts

The `Hashable` concept represents objects that can be normalized to a type-level hash.

In day to day programming, hashes are very important as a way to efficiently lookup objects in maps. While the implementation of maps is very different, the same idea of using hashes for efficient lookup applies in metaprogramming. The `Hashable` concept represents objects that can be summarized (possibly with loss of information) to a type, in a way suitable for use in hash-based data structures. Of course, in order for a hash to be well-behaved, it must obey some laws that are explained below.

## Minimal complete definition

`hash`, satisfying the laws below

## Laws

First, `hana::hash` must return a `hana::type`. Furthermore, for any two `Hashable` objects `x` and `y`, it must be the case that

x == y implies hash(x) == hash(y)

where `==` denotes `hana::equal`. In other words, any two objects that compare equal (with `hana::equal`) must also have the same hash. However, the reverse is not true, and two different objects may have the same hash. This situation of two different objects having the same hash is called a collision.

## Concrete models

`hana::integral_constant`, `hana::type`, `hana::string`

## Free model for <tt>IntegralConstant</tt>s

Any `IntegralConstant` is `Hashable`, by normalizing its value to a `hana::integral_constant`. The type of the value held in the normalized `integral_constant` is `unsigned long long` for unsigned integral types, and `signed long long` for signed integral types.