Memoize exception_lines in ExceptionPresenter

The method failure_lines uses exception_lines 3 times, which uses exception.message.
In some situation, exception.message can be slow (when inspect is called on a big object for example).
Without caching, this delay which can become considerable will be amplified by 3x.

lib/rspec/core/formatters/exception_presenter.rb

@@ -175,12 +175,14 @@ def failure_slash_error_lines
         end
 
         def exception_lines
-          lines = []
-          lines << "#{exception_class_name}:" unless exception_class_name =~ /RSpec/
-          encoded_string(exception.message.to_s).split("\n").each do |line|
-            lines << (line.empty? ? line : "  #{line}")
+          @exception_lines ||= begin
+            lines = []
+            lines << "#{exception_class_name}:" unless exception_class_name =~ /RSpec/
+            encoded_string(exception.message.to_s).split("\n").each do |line|
+              lines << (line.empty? ? line : "  #{line}")
+            end
+            lines
           end
-          lines
         end
 
         def extra_failure_lines