Add syntax for Eq, Order, and PartialOrder.

As I said previously, Algebra currently provides no syntax
so we get to do that here.

core/src/main/scala/cats/syntax/all.scala

@@ -6,12 +6,15 @@ trait AllSyntax
     with CoFlatMapSyntax
     with ComonadSyntax
     with ContravariantSyntax
+    with EqSyntax
     with FlatMapSyntax
     with FoldableSyntax
     with FunctorSyntax
     with InvariantSyntax
     with MonadCombineSyntax
     with MonadFilterSyntax
+    with OrderSyntax
+    with PartialOrderSyntax
     with ProfunctorSyntax
     with SemigroupSyntax
     with ShowSyntax

core/src/main/scala/cats/syntax/eq.scala

@@ -0,0 +1,13 @@
+package cats
+package syntax
+
+trait EqSyntax {
+  // TODO: use simulacrum instances eventually
+  implicit def eqSyntax[A: Eq](a: A) =
+    new EqOps[A](a)
+}
+
+class EqOps[A](lhs: A)(implicit A: Eq[A]) {
+  def ===(rhs: A): Boolean = A.eqv(lhs, rhs)
+  def =!=(rhs: A): Boolean = A.neqv(lhs, rhs)
+}

core/src/main/scala/cats/syntax/order.scala

@@ -0,0 +1,14 @@
+package cats
+package syntax
+
+trait OrderSyntax {
+  // TODO: use simulacrum instances eventually
+  implicit def orderSyntax[A: Order](a: A) =
+    new OrderOps[A](a)
+}
+
+class OrderOps[A](lhs: A)(implicit A: Order[A]) {
+  def compare(rhs: A): Int = A.compare(lhs, rhs)
+  def min(rhs: A): A = A.min(lhs, rhs)
+  def max(rhs: A): A = A.max(lhs, rhs)
+}

core/src/main/scala/cats/syntax/package.scala

@@ -7,12 +7,15 @@ package object syntax {
   object coflatMap extends CoFlatMapSyntax
   object comonad extends ComonadSyntax
   object contravariant extends ContravariantSyntax
+  object eq extends EqSyntax
   object flatMap extends FlatMapSyntax
   object foldable extends FoldableSyntax
   object functor extends FunctorSyntax
   object invariant extends InvariantSyntax
   object monadCombine extends MonadCombineSyntax
   object monadFilter extends MonadFilterSyntax
+  object order extends OrderSyntax
+  object partialOrder extends PartialOrderSyntax
   object profunctor extends ProfunctorSyntax
   object semigroup extends SemigroupSyntax
   object show extends ShowSyntax

core/src/main/scala/cats/syntax/partialOrder.scala

@@ -0,0 +1,20 @@
+package cats
+package syntax
+
+trait PartialOrderSyntax {
+  // TODO: use simulacrum instances eventually
+  implicit def partialOrderSyntax[A: PartialOrder](a: A) =
+    new PartialOrderOps[A](a)
+}
+
+class PartialOrderOps[A](lhs: A)(implicit A: PartialOrder[A]) {
+  def >(rhs: A): Boolean = A.gt(lhs, rhs)
+  def >=(rhs: A): Boolean = A.gteqv(lhs, rhs)
+  def <(rhs: A): Boolean = A.lt(lhs, rhs)
+  def <=(rhs: A): Boolean = A.lteqv(lhs, rhs)
+
+  def partialCompare(rhs: A): Double = A.partialCompare(lhs, rhs)
+  def tryCompare(rhs: A): Option[Int] = A.tryCompare(lhs, rhs)
+  def pmin(rhs: A): Option[A] = A.pmin(lhs, rhs)
+  def pmax(rhs: A): Option[A] = A.pmax(lhs, rhs)
+}