Add spec for Ord

[gabejohnson]

Motivated by sanctuary-js/sanctuary#353 I think it makes sense to add Ord.

From purescript-prelude

-- | The `Ord` type class represents types which support comparisons with a
-- | _total order_.
-- |
-- | `Ord` instances should satisfy the laws of total orderings:
-- |
-- | - Reflexivity: `a <= a`
-- | - Antisymmetry: if `a <= b` and `b <= a` then `a = b`
-- | - Transitivity: if `a <= b` and `b <= c` then `a <= c`
class Eq a <= Ord a where
  compare :: a -> a -> Ordering

This may require specifying Ordering as well (though I haven't seen much movement on spec'ing data types #185):

From purescript-prelude

-- | The `Ordering` data type represents the three possible outcomes of
-- | comparing two values:
-- |
-- | `LT` - The first value is _less than_ the second.
-- | `GT` - The first value is _greater than_ the second.
-- | `EQ` - The first value is _equal to_ the second.
data Ordering = LT | GT | EQ

Specifying Ord would allow generalizing several comparison (lt and friends, min & max) as well as sorting functions (sort, sortBy).

[davidchambers]

To avoid specifying Ordering we could use Number (though I'd prefer to limit the values to -1, 0, and 1).

[gabejohnson]

@davidchambers that would allow us to state the laws without referencing functions not defined in the spec. Though, by limiting the allowable values of Number you are still specifying. You're just not giving a name to the specification.

[gabejohnson]

We could just punt by adding a pointer to the 262 spec.

[gcanti]

FYI in fp-ts I'm using Ordering and Ord borrowed from PureScript

• Ordering
• Ord