Show left and right hand values when equality fails

For assertions that contain function calls, displaying just the
expression is not helpful. For e.g.

  Assertion failed
  String.upcase("foo") == "Foo"

The learner is given no extra information to help them learn. Sure they
might consult the documentation (and should!), but perhaps a better
experience would be for us to inform the learner about the values it
encountered.

  Assertion failed
  String.upcase("foo") == "Foo"
  left:  "FOO"
  right: "Foo"

Learner: 💡 ah hah! it upcases the whole string! 💫

lib/display/colours.ex

@@ -2,6 +2,7 @@ defmodule Display.Paint do
   def red(str), do: painter().red(str)
   def cyan(str), do: painter().cyan(str)
   def green(str), do: painter().green(str)
+  def yellow(str), do: painter.yellow(str)
 
   defp painter do
     case Mix.env do
@@ -17,6 +18,7 @@ defmodule Display.Colours do
   def red(str), do: colourize(ANSI.red, str)
   def cyan(str), do: colourize(ANSI.cyan, str)
   def green(str), do: colourize(ANSI.green, str)
+  def yellow(str), do: colourize(ANSI.yellow, str)
 
   defp colourize(color, message) do
     Enum.join([color, message, ANSI.reset], "")