Add Fix and FixInstances

Author: pakoito

kategory-recursion/src/main/kotlin/kategory/recursion/data/Fix.kt

@@ -0,0 +1,12 @@
+package kategory
+
+@higherkind data class Fix<F>(val FF: Functor<F>, val unfix: Fix<HK<F, HK<FixHK, F>>>) : FixKind<F> {
+
+    fun projectT(): HK<ComposedType<FixHK, F>, FixKind<F>> = unfix.lift()
+
+    companion object {
+        inline fun <reified F> embedT(compFG: HK<ComposedType<ComposedType<FixHK, F>, FixHK>, F>, FF: Functor<F> = functor<F>()): FixKind<F> =
+                Fix(FF, compFG.lower().lower().ev())
+
+    }
+}

kategory-recursion/src/main/kotlin/kategory/recursion/instances/FixInstances.kt

@@ -0,0 +1,9 @@
+package kategory
+
+interface FixInstances<F> : Birecursive<FixHK, F> {
+    override fun projectT(fg: HK<FixHK, F>): HK<ComposedType<FixHK, F>, FixKind<F>> =
+            fg.ev().projectT()
+
+    override fun embedT(compFG: HK<ComposedType<ComposedType<FixHK, F>, FixHK>, F>): FixKind<F> =
+            Fix(FG(), compFG.lower().lower().ev())
+}