Define Math module

Support `Math.factor` and cover it with tests.

assessments/semiprimes/server/lib/math.ex

@@ -0,0 +1,26 @@
+defmodule Math do
+  @moduledoc """
+  Math utilities.
+  """
+  alias Extras
+
+  @doc """
+  Returns the prime factors for `n`.
+
+  ## Examples
+
+      iex> Math.factor(15)
+      [3, 5]
+
+  """
+  def factor(1), do: []
+
+  def factor(n) do
+    Extras.range(2, n - 1)
+    |> Enum.find(&(rem(n, &1) == 0))
+    |> case do
+      nil -> [n]
+      x -> [x | factor(div(n, x))]
+    end
+  end
+end