Add missing factories similar to existing factories

kernel/src/main/scala/cats/kernel/Band.scala

@@ -14,4 +14,11 @@ object Band extends SemigroupFunctions[Band] {
    * Access an implicit `Band[A]`.
    */
   @inline final def apply[@sp(Int, Long, Float, Double) A](implicit ev: Band[A]): Band[A] = ev
+
+  /**
+   * Create a `Band` instance from the given function.
+   */
+  @inline def instance[A](cmb: (A, A) => A): Band[A] = new Band[A] {
+    override def combine(x: A, y: A): A = cmb(x, y)
+  }
 }

kernel/src/main/scala/cats/kernel/BoundedSemilattice.scala

@@ -11,4 +11,13 @@ object BoundedSemilattice extends SemilatticeFunctions[BoundedSemilattice] {
    */
   @inline final def apply[@sp(Int, Long, Float, Double) A](implicit ev: BoundedSemilattice[A]): BoundedSemilattice[A] =
     ev
+
+  /**
+   * Create a `BoundedSemilattice` instance from the given function and empty value.
+   */
+  @inline def instance[A](emptyValue: A, cmb: (A, A) => A): BoundedSemilattice[A] = new BoundedSemilattice[A] {
+    override val empty: A = emptyValue
+
+    override def combine(x: A, y: A): A = cmb(x, y)
+  }
 }

kernel/src/main/scala/cats/kernel/CommutativeMonoid.scala

@@ -15,4 +15,13 @@ object CommutativeMonoid extends MonoidFunctions[CommutativeMonoid] {
    * Access an implicit `CommutativeMonoid[A]`.
    */
   @inline final def apply[A](implicit ev: CommutativeMonoid[A]): CommutativeMonoid[A] = ev
+
+  /**
+   * Create a `CommutativeMonoid` instance from the given function and empty value.
+   */
+  @inline def instance[A](emptyValue: A, cmb: (A, A) => A): CommutativeMonoid[A] = new CommutativeMonoid[A] {
+    override val empty: A = emptyValue
+
+    override def combine(x: A, y: A): A = cmb(x, y)
+  }
 }

kernel/src/main/scala/cats/kernel/CommutativeSemigroup.scala

@@ -15,4 +15,11 @@ object CommutativeSemigroup extends SemigroupFunctions[CommutativeSemigroup] {
    * Access an implicit `CommutativeSemigroup[A]`.
    */
   @inline final def apply[A](implicit ev: CommutativeSemigroup[A]): CommutativeSemigroup[A] = ev
+
+  /**
+   * Create a `CommutativeSemigroup` instance from the given function.
+   */
+  @inline def instance[A](cmb: (A, A) => A): CommutativeSemigroup[A] = new CommutativeSemigroup[A] {
+    override def combine(x: A, y: A): A = cmb(x, y)
+  }
 }

kernel/src/main/scala/cats/kernel/Monoid.scala

@@ -101,4 +101,13 @@ object Monoid extends MonoidFunctions[Monoid] {
    * Access an implicit `Monoid[A]`.
    */
   @inline final def apply[A](implicit ev: Monoid[A]): Monoid[A] = ev
+
+  /**
+   * Create a `Monoid` instance from the given function and empty value.
+   */
+  @inline def instance[A](emptyValue: A, cmb: (A, A) => A): Monoid[A] = new Monoid[A] {
+    override val empty: A = emptyValue
+
+    override def combine(x: A, y: A): A = cmb(x, y)
+  }
 }

kernel/src/main/scala/cats/kernel/Semilattice.scala

@@ -66,4 +66,11 @@ object Semilattice extends SemilatticeFunctions[Semilattice] {
    * Access an implicit `Semilattice[A]`.
    */
   @inline final def apply[@sp(Int, Long, Float, Double) A](implicit ev: Semilattice[A]): Semilattice[A] = ev
+
+  /**
+   * Create a `Semilattice` instance from the given function.
+   */
+  @inline def instance[A](cmb: (A, A) => A): Semilattice[A] = new Semilattice[A] {
+    override def combine(x: A, y: A): A = cmb(x, y)
+  }
 }