Implement example lookup by index

This will be used later for executing tests in random order.

src/spectator/example.cr

@@ -34,6 +34,11 @@ module Spectator
       1
     end
 
+    # Retrieve the current example.
+    def [](index : Int)
+      self
+    end
+
     # String representation of the example.
     # This consists of the groups the example is in and the description.
     # The string can be given to end-users to identify the example.

src/spectator/example_component.cr

@@ -10,5 +10,8 @@ module Spectator
 
     # The number of examples in this instance.
     abstract def example_count : Int
+
+    # Lookup the example with the specified index.
+    abstract def [](index : Int) : Example
   end
 end

src/spectator/example_group.cr

@@ -31,6 +31,25 @@ module Spectator
       children.sum(&.example_count)
     end
 
+    def [](index : Int) : Example
+      raise IndexError.new if index < 0
+      offset = index
+      found = children.find do |child|
+        count = child.example_count
+        if offset < count
+          true
+        else
+          offset -= count
+          false
+        end
+      end
+      if found
+        found[offset]
+      else
+        raise IndexError.new
+      end
+    end
+
     # TODO: Remove this method.
     def all_examples
       Array(Example).new(example_count).tap do |array|